squish-pty: Flush buffered data from pty to stdout when pty's slave closed

squish-pty: Flush buffered data from pty to stdout when pty's slave closed

When the slave side of the pty is closed, the "read" system call returns
EIO.  In this situation, squish-pty failed to flush any output that it
already had in its buffer (up to 16 bytes) to stdout.  This could cause
the "Powering off..." message printed by Pintos just before exiting to
be missing from the output.

This usually didn't happen, because there are two other exit paths from
the relay() function that does most of squish-pty's work.  The most common
exit path was (evidently) the one which detected that the child process
had died, which did properly flush the buffer.  (The third path was wrong
too, and this patch fixes that one too.)

This patch fixes the problem by using a single exit path from relay() that
always reads any remaining input from the pty (if it is open) and flushes
it to stdout.

Reported by Borja Sotomayor <borja@cs.uchicago.edu>.