Fix 'mmap' lib/filehand.t (ungetc) test fail.

Fix 'mmap' lib/filehand.t (ungetc) test fail.
Make 'unix' pass most tests
 - do unread by temporary push of layer ("pending") holding unread chars
 - fast_gets is now based on per-handle flag
 - relax one of io/pipe.t tests to allow print to fail and close to
   succeed so that it passes on unbuffered "unix" layer.
Remaining fail is sporadic and a genuine race condition between
parent/child in fork test.

p4raw-id: //depot/perlio@7942