Avoid misleading error on srun --pty exit

There was an error reported by slurmstepd trying to read the window
size from srun when srun was exiting. This eliminates that spurious
error message.
Restore srun --pty resize ability

It seems to have been broken for some time due to logic blocking
SIGWINCH being removed from srun, which could result in the
signal being sent to any thread, not the thread designed to handle
window resizing.