This allows us to actually mount and run the system inside a container for
more advanced debugging.

I've successfully booted my systemd, logged in, had a fiddle and then
powered it off again. There are some delays and it's requried to umount
the /mnt/proc which seems to get automatically mounted before running
nspawn. Also audit support has to be turned off in the kernel too to
allow a full boot to work properly (otherwise the authentication in
the container doesn't work. This is a known problem in the kernel and
will be fixed eventually.

This commit converts the rescue system to systemd rather than the legacy
sysvinit.

Besides being faster, one primary advantage of running systemd here is
that we can "boot" the system we're inspecting via systemd-nspawn
to properly test it.

Note: Bits that don't work:
 * nspawn
 * old dracut initqueue udev rules in /etc/ are not cleaned out (drop in
   support error?)
 * the gui may loop differently now

The main change here is not remounting a fresh /run. Dracut does this
for us and we actually need to keep this one as the initial udev
database (when udev is started by dracut) is stored here and thus
mounting an empty fs could see us lose some metadata.

We no longer touch the /bin, /sbin or /lib[64] symlinks as these are
already relative symlinks into /usr and simply bind mounting /usr is
enough.