Implement FLUSHO

Turn FLUSHO on/off with ^O (or whatever VDISCARD is). Honor that to
throw away output quickly. This tries to remain true to 4.4BSD
behavior (since that was the origin of this feature), with any
corrections NetBSD has done. Since the implemenations are a little
different, though, some edge conditions may be handled differently.

Reviewed by: kib, kevans
Differential Review: https://reviews.freebsd.org/D26148