Use the sinit conventions for init

This should make it easier to switch out `init` for other versions,
although the `getty` config still needs to be removed.

- use `/bin/rc.init` for start
- use `rc.shutdown` for stop
- make `rc.init` run other code.
- merge `containers` and `containerd` startup code

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

pkg/init/bin/rc.shutdown

@@ -0,0 +1,13 @@
+#!/bin/sh
+
+[ "$1" = "reboot" ] && exec /sbin/reboot
+
+# poweroff
+
+/usr/sbin/killall5 -15
+/bin/sleep 5
+/usr/sbin/killall5 -9
+/sbin/swapoff -a
+/bin/echo "Unmounting filesystems"
+/bin/umount -a -r
+/sbin/poweroff -f