Technically, because we don't actually switchroot into this filesystem
we don't need some of the directories and symlinks (as they are already
present in the initrd).

But with a long term view to making this a "real" filesystem that we DO
switchroot into, it makes sense to try and keep things vaguely valid.

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