Use new /dev/loop-control device to create loop devices

Use new /dev/loop-control device to create loop devices

See

 http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=770fe30a46a12b6fb6b63fbe1737654d28e84844

for the commit that introduced this feature. This is available as of
Linux 3.1.

Second, since the kernel now (virtually) supports an unlimited number
of loop devices, allow local active users to do this without having to
authenticate.

Signed-off-by: David Zeuthen <davidz@redhat.com>