libmount: Support unmount FUSE mounts

libmount: Support unmount FUSE mounts

FUSE mounts don't need an fstab entry to be unmounted.
This checks if a mount is a FUSE mount before checking for
the fstab entry, and if so returns success.

[kzak@redhat.com: - use libmount tools for mount options
                  - use namespace switches
                  - cleanup code

 The requirement is user=<username> or user_id=<uid> in /proc/self/mountinfo
 for fuse filesystem. The logic is the same as for user= mount options, but in
 this case it is not maintained by libmount in userspace, but by fuse FS in kernel.]

Co-Author: Karel Zak <kzak@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>