systemctl: restore "systemctl reboot ARG" functionality

systemctl: restore "systemctl reboot ARG" functionality

Commit d85515edcf9700dc068201ab9f7103f04f3b25b2 changed logic how reboot is
executed. That commit changed behavior to use emergency action reboot code path
to perform the reboot.

This inadvertently broke rebooting with argument:
$ systemctl reboot custom-reason

Restore original behavior so that if reboot service unit similar to
systemd-reboot.service is executed it is possible to override reboot reason
with "systemctl reboot ARG".

When "systemctl reboot ARG" is executed ARG is placed in file
/run/systemd/reboot-param and reboot is issued using logind's Reboot
dbus-service.

If RebootArgument is specified in systemd-reboot.service it takes precedence
over what systemctl sets.

Fixes: #11828