RFR [13] 8225426: Replace plain with system-default in Socket and ServerSocket

[Chris Hegarty]

Another minor followup that we agreed to address after the integration
of 8221481 (Reimplement the Legacy Socket API).

Prior to 8221481, there was just a single default SocketImpl, the
"plain" socket impl. There are now two, PlainSocketImpl and
NioSocketImpl, the latter being the default. The former can be selected
as the default by setting the `jdk.net.usePlainSocketImpl` system
property, further information an be found in the Implementation Note of
`java.net.SocketImpl`.

The term "plain" is mentioned in a few places in the specification of
j.n.Socket and j.n.ServerSocket. These should be updated to
"system-default", which is the agreed term post 8221481.


src/java.base/share/classes/java/net/ServerSocket.java

     /**
      * Creates a server socket, bound to the specified port. A port number
      * of {@code 0} means that the port number is automatically
      * allocated, typically from an ephemeral port range. This port
      * number can then be retrieved by calling {@link #getLocalPort getLocalPort}.
      * <p>
      * The maximum queue length for incoming connection indications (a
      * request to connect) is set to {@code 50}. If a connection
      * indication arrives when the queue is full, the connection is refused.
      * <p>
      * If the application has specified a server socket factory, that
      * factory's {@code createSocketImpl} method is called to create
-     * the actual socket implementation. Otherwise a "plain" socket is created.
+     * the actual socket implementation. Otherwise a system-default
+     * {@code SocketImpl} is created.
      * <p>
      * If there is a security manager,
      * its {@code checkListen} method is called
      * with the {@code port} argument
      * as its argument to ensure the operation is allowed.
      * This could result in a SecurityException.
      *...
      */
     public ServerSocket(int port) throws IOException

     /**
      * Creates a server socket and binds it to the specified local port
      * number, with the specified backlog.
      * A port number of {@code 0} means that the port number is
      * automatically allocated, typically from an ephemeral port range.
      * This port number can then be retrieved by calling
      * {@link #getLocalPort getLocalPort}.
      * <p>
      * The maximum queue length for incoming connection indications (a
      * request to connect) is set to the {@code backlog} parameter. If
      * a connection indication arrives when the queue is full, the
      * connection is refused.
      * <p>
      * If the application has specified a server socket factory, that
      * factory's {@code createSocketImpl} method is called to create
-     * the actual socket implementation. Otherwise a "plain" socket is created.
+     * the actual socket implementation. Otherwise a system-default
+     * {@code SocketImpl} is created.
      * <p>
      * ...
      */
      public ServerSocket(int port, int backlog) throws IOException


src/java.base/share/classes/java/net/Socket.java

     /**
      * Creates a stream socket and connects it to the specified port
      * number on the named host.
      * <p>
      * If the specified host is {@code null} it is the equivalent of
      * specifying the address as
      * {@link java.net.InetAddress#getByName InetAddress.getByName}{@code (null)}.
      * In other words, it is equivalent to specifying an address of the
      * loopback interface. </p>
      * <p>
      * If the application has specified a server socket factory, that
      * factory's {@code createSocketImpl} method is called to create
-     * the actual socket implementation. Otherwise a "plain" socket is created.
+     * the actual socket implementation. Otherwise a system-default
+     * {@code SocketImpl} is created.
      * <p>
      * If there is a security manager, its
      * {@code checkConnect} method is called
      * with the host address and {@code port}
      * as its arguments. This could result in a SecurityException.
      *
      * ...
      */
     public Socket(String host, int port) throws UnknownHostException, IOException


     /**
      * Creates a stream socket and connects it to the specified port
      * number at the specified IP address.
      * <p>
      * If the application has specified a socket factory, that factory's
      * {@code createSocketImpl} method is called to create the
-     * actual socket implementation. Otherwise a "plain" socket is created.
+     * actual socket implementation. Otherwise a system-default
+     * {@code SocketImpl} is created.
      * <p>
      * If there is a security manager, its
      * {@code checkConnect} method is called
      * with the host address and {@code port}
      * as its arguments. This could result in a SecurityException.
      *
      * ...
      */
     public Socket(InetAddress address, int port) throws IOException

     /**
      * Creates a stream socket and connects it to the specified port
      * number on the named host.
      * <p>
      * If the specified host is {@code null} it is the equivalent of
      * specifying the address as
      * {@link java.net.InetAddress#getByName InetAddress.getByName}{@code (null)}.
      * In other words, it is equivalent to specifying an address of the
      * loopback interface. </p>
      * <p>
      * If the stream argument is {@code true}, this creates a
      * stream socket. If the stream argument is {@code false}, it
      * creates a datagram socket.
      * <p>
      * If the application has specified a server socket factory, that
      * factory's {@code createSocketImpl} method is called to create
-     * the actual socket implementation. Otherwise a "plain" socket is created.
+     * the actual socket implementation.Otherwise a system-default
+     * {@code SocketImpl} is created.
      * <p>
      * If there is a security manager, its
      * {@code checkConnect} method is called
      * with the host address and {@code port}
      * as its arguments. This could result in a SecurityException.
      * <p>
      * If a UDP socket is used, TCP/IP related socket options will not apply.
      *
      * ...
      */
     @Deprecated
     public Socket(String host, int port, boolean stream)

     /**
      * Creates a socket and connects it to the specified port number at
      * the specified IP address.
      * <p>
      * If the stream argument is {@code true}, this creates a
      * stream socket. If the stream argument is {@code false}, it
      * creates a datagram socket.
      * <p>
      * If the application has specified a server socket factory, that
      * factory's {@code createSocketImpl} method is called to create
-     * the actual socket implementation. Otherwise a "plain" socket is created.
+     * the actual socket implementation. Otherwise a system-default
+     * {@code SocketImpl} is created.
      *
      * <p>If there is a security manager, its
      * {@code checkConnect} method is called
      * with {@code host.getHostAddress()} and {@code port}
      * as its arguments. This could result in a SecurityException.
      * <p>
      * If UDP socket is used, TCP/IP related socket options will not apply.
      *
      * ...
      */
     @Deprecated
     public Socket(InetAddress host, int port, boolean stream)

-Chris.