libmount: fix support of comma-separated fs types lists

libmount: fix support of comma-separated fs types lists

	# grep cdrom /etc/fstab
	/dev/sr0 /media/cdrom udf,iso9660 ro,noauto,user,utf8 0 0
	# mount /media/cdrom
	mount: unknown filesystem type 'udf,iso9660'

	# mount -t udf,iso9660 /dev/sr0 /media/cdrom
	mount: /dev/sr0 is write-protected, mounting read-only

[kzak@redhat.com: - add some comments
                  - don't try to found external helpers for the types]

Signed-off-by: Gleb Fotengauer-Malinovskiy <glebfm@altlinux.org>
Signed-off-by: Karel Zak <kzak@redhat.com>