Integrate perlio:

[  7942]
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-link: @7942 on //depot/perlio: 5e2ab84baab5f2372dc1ffda47d5b89faa2613cd

p4raw-id: //depot/perl@7947