8231921: SelectorImpl.register does not need to set the attachment when it is null

[Alan Bateman]

This is a small improvement to SelectableChannel.register to avoid 
needing to getAndSet the attachment when initially registered without an 
attachment.  While in the area, I've replaced the use of 
AtomicReferenceFieldUpdater with a VarHandle. The changes are simple:

http://cr.openjdk.java.net/~alanb/8231921/webrev/index.html

-Alan