systemctl: let's emphasize a bit that halt/poweroff/reboot/shutdown are compat commands

systemctl: let's emphasize a bit that halt/poweroff/reboot/shutdown are compat commands

Let's try to make clear that these calls really should not be extended
anymore, but remain as the compat glue they are but not more.

Anything new should really be added to systemctl poweroff/halt/reboot,
which is actually defined and owned by us.

This is added in light of a9c3cc8db02dc36d41b17d0bbf1e02500046e4ce which
really shouldn't have been added I am sure.

This adds two things: a note to the --help text that people use the
relevant systemctl commands instead (as they are a lot more powerful,
for example give you inhibitor and boot loader control, kexec, and so
on). And a note to developers that they stop adding new stuff to the
compat interfaces.