New commands: 'mkmountpoint' and 'rmmountpoint'

New commands: 'mkmountpoint' and 'rmmountpoint'

These specialized commands are used to create additional mountpoints
before mounting filesystems.  They are only used where you want to
mount several unrelated or read-only filesystems together, and need
additional care to use correctly.

Here is how to use these calls to unpack the "Russian doll" nest
of a Fedora 11 live CD:

 add-ro Fedora-11-i686-Live.iso
 run
 mkmountpoint /cd
 mkmountpoint /squash
 mkmountpoint /ext3
 mount /dev/sda /cd
 mount-loop /cd/LiveOS/squashfs.img /squash
 mount-loop /squash/LiveOS/ext3fs.img /ext3

The inner filesystem is now unpacked under the /ext3 mountpoint.