PlugStackConfig and UsePAM. Update some related documentation.

gid must be the root user.  Therefore we need to reclaim the effective
gid in _become_user() before dropping real, effective, and saved gids.

Likewise for setreuid().

validate the memory size.

PAM to support SLURM (under the UsePAM) option.

for consistency across slurm

for consistency across slurm

Change avoids this error:
  expected integer but got "08" (looks like invalid octal number)

No changes in logic.

    from "MM/DD HH:MM:SS" to "YYYY-MM-DD HH:MM:SS", conforming with the ISO8601
    standard format.

The extern name "fini" is reserved for a plugin termination function.

Possible race condition on mail sending while slurmctld is shutting down.