o src/slurmd : fail job launch if shared memory is filled.

 o src/slurmd : do not bother to sleep full timeout in rpc_timelimit
   if the job step goes away on SIGTERM.
 o src/common/slurm_auth.c : get default plugin dir from SLURM_PLUGIN_DIR
   rather that always "/usr/local/lib/slurm"
 o other small fixes