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 locked into memory.

 * We unpack the initramfs to /run/initramfs at *boot* time: sadly, I was not
   able to have it unpacked reliably in udev-watchdog-wrapper when the boot
   medium is ejected, so we'll use a little bit more RAM (instead of locking the
   compressed initramfs into memory, we're storing the uncompressed one there)
   and probably slow down the boot a bit, in order to make emergency shutdown
   robust. Note, however, that we save some of the RAM used by the uncompressed
   initramfs by deleting the worst offenders (kernel modules).

 * For now the whole procedure is quite noisy on the screen: the pre-shutdown
   hook runs under "set -x", doesn't run "clear", and spits out lots of
   debugging information. The goal is to enable users to provide useful
   debugging data if they have problems with emergency shutdown. Once we have
   shipped this code in a few releases and trust it's robust enough, we can
   surely reconsider and polish the UX by making the output less noisy.

 * We use absolute paths in many places to avoid $PATH lookup which might
   fail if the root filesystem is not there anymore.