losetup: add a new option -s

losetup: add a new option -s

The losetup provides an -f option to discover an unused loop device.  As
implemented, it is racy. The problem is that -f prints the loop device
to stdout only when used standalone.  This means a script has to do:

 lodev=$(losetup -f)
 losetup $lodev $filename

which is racy if another script may be running.

This patch add a new option '-s'. The '-s' option prints device name
if the -f option and a file argument are present. For example:

 lodev=$(losetup -f -s $filename)

Signed-off-by: Karel Zak <kzak@redhat.com>