8235193: JDK-8235193,(dc) Remove JNI overhead from DatagramChannel.send implementation

[Alan Bateman]

JDK-8234805 removed the JNI upcalls from DatagramChannel.receive. This 
is a follow-up to improve the send method by eliminating the use of 
NET_InetAddressToSockaddr (lots of JNI calls) and replacing it with code 
to populate the socket address using the NativeSocketAddress 
implementation class added in JDK-8234805.

The webrev with the changes is here.
     http://cr.openjdk.java.net/~alanb/8235193/webrev/

-Alan