Removed call to fpurge() and its platform-specific emulations

Removed call to fpurge() and its platform-specific emulations
from discard.c.  discard() is called on stdout from mhl(1) and
msh(1) in interrupt handlers, and post(8) but only with -debug.
tcflush() should do the purge when stdout is a terminal.  If
it's a file or pipe, well, maybe some garbage could get flushed
out.  And it's called on the streams that post uses to talk to
its smtp server, so removed those calls because they no longer
do anything.  And removed LINUX_STDIO define from configure.ac.