losetup: fix util-linux compatibility

Added -a support. Also made sure -f works as follows:

losetup [-r] [-o offset] {-f|loopdev} file

Removed support for 'losetup -r' with no arguments.

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Merge git://busybox.net/busybox