slurmd: Call spank prolog and epilog hooks

Call spank_job_prolog() and spank_job_epilog() at prolog/epilog
time by invoking "slurmstepd spank [prolog|epilog]"

The prolog and epilog spank plugin hooks are not called within the
virtual address space of slurmd for at least a couple of reasons,
including

 1. Plugins dlopened in the address space of slurmd cannot be dlopened
   a second time. Therefore, static and global state in the DSO may
   be "dirty" in that some state may be preserved from the last epilog
   or prolog call, or even from the slurmd_init callback.

 2. The prolog and epilog need to be guaranteed reentrant. The safest
   way to guarantee this is to ensure prolog/epilog hooks are called
   from a separate address space.

 3. To satisfy "principle of least surprise" we want to have new plugins
   installed run their prolog/epilog hooks on the next job, just as
   if an update to the prolog/epilog script was made. The only way to
   guarantee this is to reload the spank plugin stack from plugstack.conf
   on each run. Because of #1 above, this needs to be done in a separate
   process.