Fix #302: add support for running scripts at shutdown

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>

src/plugin.h

@@ -82,6 +82,8 @@
 								\
 	/* Shutdown hooks, runlevel [06] */			\
 	CHOOSE(HOOK_SHUTDOWN,        "hook/sys/shutdown"),	\
+	CHOOSE(HOOK_SVC_DN,          "hook/svc/down"),		\
+	CHOOSE(HOOK_SYS_DN,          "hook/sys/down"),		\
 	CHOOSE(HOOK_MAX_NUM,         "nop")			\
 }