FileSock is bidirectional channel associated with opened file.

FileSock provides streaming write/read operations for filesystem server that
are correspondingly matched with read/write operations on filesystem user side.

WCFS will use FileSock to implement exchange over .wcfs/zhead and,
later, head/watch files.

Some preliminary history:

b17aeb8c    X Change FileSock to use xio.Pipe which is io.Pipe + support for IO cancellation