umount: add --fake option to umount(8)

umount: add --fake option to umount(8)

Add --fake option to umount(8), which omits calling the actual umount
syscall (and the loop device deletion) but modifies /etc/mtab.  This
is similar to the -f or --fake option to mount(8).

This would allow some simplifications in fuse by allowing it to call
the umount syscall and letting umount(8) just update mtab.

[kzak@redhat.com: - initialize 'res' variable in umount_one() ]

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Karel Zak <kzak@redhat.com>