Since different distros may or may not use vol_id in udev, and blkid
is generally replacing vol_id, abstract them out into a function which
tries to use vol_id first and blkid second, on the assumption that
blkid can take over for vol_id if vol_id is no longer there.
With the /check infrastucture in place, dmraid is missing a check file.
Patch is built on top of Harald Hoyers merge tree
(git://git.surfsite.org/pub/git/dracut.git)