Return to the initramfs (unpacked in /run/initramfs) on shutdown (refs: #12428, #12354, Debian#778849).

… otherwise the aufs read-write (tmpfs) branch, among possibly other things,
can't be properly unmounted and its content remains in memory.

Notes:

 * We have to handle some unmounting ourselves in initramfs-pre-shutdown-hook:
   systemd-shutdown doesn't manage to unmount the aufs read-write
   branch (/oldroot/lib/live/mount/overlay) as it is needed by the
   aufs (/oldroot) filesystem, and reciprocally it cannot unmount /oldroot as it
   is kept busy by /oldroot/lib/live/mount/*. So we disentangle this mess
   ourselves. And we have to manually empty the aufs read-write (tmpfs) branch,
   otherwise for some reason its content remains in memory. This code will of
   course need to be adapted for overlayfs some day.

 * We lock /bin/kill in memory: apparently systemd-exit.service needs it.

 * We remount /run on shutdown *before* dropping caches, just in case dropping
   caches removes what we've loc...