main: Cleanly quit on SIGTERM

main: Cleanly quit on SIGTERM

Now that we are no longer unconditionally opting out of getting terminated,
we get send a SIGTERM when transitioning from the rootfs back to the initrd
on poweroff/reboot.

Catch this SIGTERM and then exit cleanly by calling the on_quit handler,
besides exiting cleanly being the right thing to do, this will also allow the
boot-splash to go idle, so that it can cleanly finish the end-animation.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>