8198754: (ch) Separate blocking and non-blocking code paths (part 2)

[Alan Bateman]

This is the follow-up to JDK-8198562 to cover the refactoring of the 
DatagramChannel and Pipe.* implementations. As with JDK-8198562, I've 
done some cleanup while in the area but I didn't want to do too much as 
it will take from the core changes. I will create a few issues in the 
JIRA for follow-up cleaned up to be done in this area, esp. where spec 
and implementation aren't aligned.

The webrev with the changes for part 2 are here:
    http://cr.openjdk.java.net/~alanb/8198754/webrev/index.html

-Alan