Enforce valid combinations of ProcTrackType and PrologFlags

PrologFlags=x11 is incompatible with proctracktype=proctrack/linuxproc
because linuxproc cannot kill processes not owned by the user, and root
owns processes spawned for handling x11 forwarding.

When using PrologFlags=contain, it is recommended to use
proctrack/cgroup or proctrack/cray because contain is mostly useful for
pam_slurm_adopt which requires proctrack/cgroup or proctrack/cray.

Bug 6824.