This makes it possible to mount unionfs directly with mount or fstab.
Example fstab entry:
none /mnt/union unionf-fuse allow-other,dirs=/source1=RW,/source2=RW
Note, the dirs= option need to be the last option, due to limitations
in the mount helper provided by unionfs-fuse. Maybe we should write a
better helper ourselves.
Also, you need to specify your branches as dirs= option, NOT as
the device, since the latest version of libmount always interprets
"=" characters in the device name as a tag (like LABEL, UUID etc)
and will try to resolve the tag before calling the mount helper.
This will of course not succeed and the mount will fail.
