try defaults if auto-detection failed in apt-cdrom

The default is to ask udev for location and mountpoints of CD-ROMs,
but the old way of specifying the mountpoint is still available and
is tried now in case udev doesn't find any CD-ROM.

It probably fails, too, but we get a bunch of error messages this
way and the user can get an idea of how to make his setup work even
if udev can't be convienced to return something useful.

Closes: #712433