losetup: improve command line option processing

losetup: improve command line option processing

The options --detach, --remove, and --set-capacity are mutually
exclusive. We can assume the device name follows the options, which is
better than assuming the device name is specified as an option's
argument. This also allows the use of the existing mutually-exclusive
check.

 # losetup --remove --detach loop0
 losetup: /dev/--detach: remove failed: Success
 losetup: /dev/loop0: remove failed: Device or resource busy

is ugly.

Reported-by: Benno Schulenberg <bensberg@telfort.nl>
Signed-off-by: Karel Zak <kzak@redhat.com>