8236246: SelectorProvider support for creating a DatagramChannel that is not interruptible

[Alan Bateman]

This is an update to the SelectorProvider implementation with two 
related parts to it:

1. The upcoming replacement of the DatagramSocket and MulticastSocket 
implementations needs a socket adaptor that is not interruptible. It 
also needs to be able to create it with built-in selector provider. This 
part of the changes makes this easy with 
DefaultSelectorProvider.get().openUninterruptibleDatagramChannel(). The 
main difference between an interruptible and uninterruptible 
DatagramChannel is that the hooks for Thread.interrupt aren't setup 
around blocking operations, instead the uninterruptible case just needs 
to check for async close at the end of blocking operations.

2. The second part is that SelectorProvider is specified to locate the 
default provider when its "provider" method is first called. The current 
implementation uses a lock to allow that initialization but it means 
that the creation of all channels with the static factory methods (the 
common case) ends up being synchronized. Moving the initialization of 
the default provider to a holder class allows this lock to be removed.

The webrev with the changes is here. I've added a test to exercise the 
(internal) openUninterruptibleDatagramChannel.

http://cr.openjdk.java.net/~alanb/8236246/webrev/

SAP folks - sun.nio.ch.DefaultSelectorProvider is changed but I have no 
way to test the AIX change. I assume your testing will catch any issues 
there.

-Alan