This adds a concept of "extrinsic" mounts. If mounts are extrinsic we
consider them managed by something else and do not add automatic
ordering against umount.target, local-fs.target, remote-fs.target.

Extrinsic mounts include API mounts such as everything below /proc,
/sys, /dev.

This avoids a crash in LXC containers where /dev/urandom is a bind mount
from the host system and unmounting it leads to an assert in systemd.

Closes: #818978