RFR 8195059: Update java.net Socket and DatagramSocket implementations to use Cleaner

[Roger Riggs]

Please review changes to replace finalizers in socket, datagram, and 
multicast networking
with Cleaner based release of the raw file descriptors.  Each 
FileDescriptor is registered
for cleanup after the raw fd (or handle) is assigned.  Normal calls to 
close unregister the
cleaner before using the current logic to close the raw fd/handle. 
Windows networking
uses fd's with the Windows socket_ API requiring a special cased Cleanable.

The tests check that the implementation objects including 
FileDescriptors are reclaimed
and for Linux that the raw fd counts are reduced as expected.

Webrev:
    http://cr.openjdk.java.net/~rriggs/webrev-net-cleanup-8195059/

Thanks, Roger