inspect: recognise Debian live images as such

Check for filesystem.squashfs also in /live, since it is where
live-build places it.
inspect: ignore special CD devices on FreeBSD install discs

/etc/fstab in installation discs of FreeBSD can have an entry pointing
to the mounted CD itself; skip it as it is done with other CD devices in
check_fstab.
inspect: improve detection of FreeBSD install discs (RHBZ#1033207).

Check for /boot/loader.rc as "install disc" detection, using it to mark
FreeBSD install discs.
Also, check for /mfsroot.gz to see whether such disc is also a live one.

See also RHBZ#1033207.