compel: error out on unknown opt or missing arg

getopt_long() prints an error message and returns '?' in cases
 - an unknown option is given
 - a required option argument is missing

In such cases, we need to show usage and exit with an error.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>