systemd-nspawn always outputs some debug messages over stderr.
Both stdout and stderr from inside the chroot are sent through
a pty to stdout.

Allows calling makechrootpkg without worrying about the architecture

Recent changes to systemd-nspawn have it take the machine name from
the chroot dir name, which isn't unique enough for our setup.

Separates the two features of mkarchroot. Provides users of the new
arch-nspawn with the full feature set of systemd-nspawn.

For example, this can be used to bind custom directories into the chroot.