8236925: (dc) Upgrade DatagramChannel socket adaptor to extend MulticastSocket

[Alan Bateman]

This is a patch to "upgrade" the DatagramChannel socket adaptor to 
extend MulticastSocket. This is one of the final changes needed to 
DatagramChannel and its socket adaptor buddy to make it easy to replace 
the legacy DatagramSocket and MulticastSocket implementation. Daniel has 
a draft JEP [1] with the all the details on this effort.

The changes should be straight-forward to review as most of the methods 
defined by MulticastSocket are easy to implement on DatagramChannel. A 
few review notes:

1. MulticastSocket does not have a protected constructor to create the 
socket with a custom DatagramSocketImpl. The DatagramSocketImpl 
mechanism is legacy and not worth adding a constructor now. So the most 
significant change in the patch is that the socket adaptor is changed to 
invoke the MulticastSocket(SocketAddress) constructor. This requires a 
special checked in the MulticastSocket and DatagramSocket constructor 
that isn't pretty but it has the advantage that the dummy 
DatagramSocketImpl can go away (it was never used anyway).

2. The DatagramSocket.impl field is changed to final. This is nice 
because it avoids accidents with lazily creating an impl (say where the 
socket adaptor doesn't override all methods). The final field means 
there are several opportunities to for cleanup but I don't want to go 
there in this patch (this is why getImpl continues to declare that it 
throws SocketException). I also don't want to complicate things for the 
JEP implementation in the sandbox.

3. The MulticastSocket spec in lacking in many areas and doesn't specify 
the exceptions or behavior for many scenarios. The joinGroup methods 
don't specify how they behave when already a member of the group. The 
leaveGroup method doesn't specify the behavior when not a member. 
Several methods don't specify the exception when invoked with null or 
other bad input. The overrides in the socket adaptor try to maintain 
compatibility with the the long standing behavior.

4. The DatagramSocket::socket spec is modified to drop the statement 
that the socket adaptor doesn't have any public methods beyond those 
defined by DatagramSocket. We need to drop the same statement from 
SocketChannel and ServerSocketChannel, something for another issue. So I 
need a CSR and that can also track the observable change that the 
returned object is a MulticastSocket.

5. One issue with the test is that is initially tickled an intermittent 
bug on macOS. The underlying setsockopt fails intermittently with ENOMEM 
when attempting to join the group. There have been sightings of this 
issue going back to JDK 7 [2]. Also sightings with the legacy 
MulticastSocket implementation. The patch has a temporary workaround to 
this issue. It's ugly but I can't duplicate it with this change. Daniel 
has been thinking about doing the same workaround while we figure out if 
there is a fix for macOS.

The webrev with the changes is here:
     http://cr.openjdk.java.net/~alanb/8236925/webrev/

-Alan

[1] https://bugs.openjdk.java.net/browse/JDK-8235674
[2] https://mail.openjdk.java.net/pipermail/net-dev/2013-July/006769.html