core: freeze execution if /etc/mtab exists

The mount monitor that was added to libmount v2.27 requires /etc/mtab to be
non-existant. As systemd now uses that functionality, we cannot monitor any
mounts anymore, and hence not support .mount units.

Systems that have /etc/mtab around as regular file are unsupported by
systemd since a long time.

This patch makes that condition fatal, so we do not boot up with
non-working mount monitor support.