This change adds support for running hook scripts at two new hook points
during the shutdown process:
- HOOK_SVC_DN: after all services and non-reserved processes have been
killed (and collected)
- HOOK_SYS_DN: after all file systems have been unmounted, *just prior*
to Finit calling reboot() to shut down or reboot the system
See doc/plugins.md for details and an example.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>