pmount does not mount CD-ROM devices

pmount is currently not able to mount iso9660 CD-ROMs when they are not
contained in /etc/fstab:

$ pmount /dev/hdb
mount: block device /dev/hdb is write-protected, mounting read-only
mount: wrong fs type, bad option, bad superblock on /dev/hdb,
       or too many mounted file systems
Error: mount failed

Some experiments show that mount fails on the particular combination of a long
filesystem list and the "rw" option. Thus, future pmount versions should try to
mount iso9660 and udf file systems separately (probably also with 'ro').

This is not a showstopper for Warty, the installer creates fstab entries for
internal CD-ROMs (it will just give problems with USB CD-ROM drives). But in the
long run, these could be handled by pmount as well.