RFR 8245194: Unix domain socket channel implementation

[Alan Bateman]

One other comment on the changes to the file system provider it should 
be okay to change UnixUserPrinipals ad its fromUid and fromGid method to 
be public. This would mean that the patch shouldn't need to add 
UnixUserGroup (the main issue is that class is that it means we end up 
with two classes with static factory methods doing the same thing).

-Alan