Add logic for job container create at alloc time

Add PrologFlags option of "Contain" to create a proctrack container at
    job resource allocation time.
At job allocation time, a slurmstepd is spawned on every allocated
    compute node in which to place external processes (e.g. PAM
    can place ssh processes into a cgroup).
This entity is accounted for and reported by sacct as "<jobid>.extern".
Some more testing and development remain, but it mostly works.