Remove the srun options -A/--allocate, -b/--batch, and -a/--attach.

That functionality is now available in salloc, sbatch, and sattach,
respectively.

Also removed the forked message process from srun.  The slurmctld will no
longer kill a job allocation just because it cannot ping srun, so it
is no longer critical to keep a message process running when the user
stops the srun process.