Avoid hangs in slurmstped due to blocking IO to pty

When pty terminal emulation is used (srun --pty), the user's pseudo
terminal has the potential to cause IO to block. Therefore, the code
must avoid connecting slurmstepd's STDERR_FILENO to user pty to avoid
a potential slurmstepd hang.
Patch from Chris Morrone, LLNL.