use nspawn to bind mount needed directories

systemd-nspawn is capable of doing this as of systemd-198. Doing this
means we can remove all of our home grown chroot mount/umount logic, as
it's all performed by pacstrap or systemd-nspawn.

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>