job_container/tmpfs: add functionality to restore NSs state after restart

container_p_restore get now the list of jobs running from the spool dir
with stepd_available.

Then, it iterates over basepath entries and, for those which seems to
have been a mount point (has .ns file), tries to mount it again.

If it succeeds (it must), and if for this mount point the job is dead,
it releases resources and tries to delete files. Remember the removal
can fail if a resource is leaked. These would be fixed if slurmd starts
after HW reboot (no kernel leaks).

Bug 11093