RFR 8058965: Remove IPv6 support from TwoStacksPlainSocketImpl

[Ivan Gerasimov]

Thank you Michael!

> Okay. I see the "part 1" in the subject line now, but maybe the bug 
> should be updated
> to define the scope of the change and we should file a separate bug 
> report then.

I created the subtask to track this part (the subject of the thread is 
changed accordingly):
https://bugs.openjdk.java.net/browse/JDK-8058965

> I'm okay with leaving the class with the current name. What about the 
> datagram socket code?
> It might make sense to include the equivalent changes there also.
>
TwoStacksPlainDatagramSocketImpl is there for two purposes: for IPv4 
only as above and for doing multicasting.
DualStackPlainDatagramSocketImpl does not have multicasting implemented 
(from the comment: "This is to
overcome the lack of behavior defined for multicasting over a dual layer 
socket by the RFC"

I'm going to see if it's possible to implement multicasting over 
DualStack socket with the current API:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms738558(v=vs.85).aspx

If it is, it should be possible to drop IPv6 part of 
TwoStacksPlainDatagramSocketImpl as well.

> There might need to be some code in net_util_md.c that needs to be 
> removed also (NET_Timeout2() function?)
>

I've just checked its content, and it all is still used somewhere.
NET_Timeout2() is called from TwoStacksPlainDatagramSocketImpl.
Once we remove/reoraganize it, all the unused functions will be erased too.

Sincerely yous,
Ivan