systemctl: make shutdown operations use irreversible jobs

systemctl: make shutdown operations use irreversible jobs

Occasionally people report problem with reboot/poweroff operations hanging in
the middle. One known cause is when a new transaction to start a unit is
enqueued while the shutdown is going on. The start of the unit conflicts with
the shutdown jobs, so they get cancelled. The failure case can be quite unpleasant,
becase getty and sshd may already be stopped.

Fix it by using irreversible jobs for shutdown (reboot/poweroff/...) actions.
This applies to commands like "reboot", "telinit 6", "systemctl reboot". Should
someone desire to use reversible jobs, they can say "systemctl start reboot.target".`