Allow use of a single path if multipath activation fails. (#1054806)

If multipath loses the race to claim the member disks and something
like lvm activates devices on one of the members, allow direct use
of that member and ignore the others. This allows configuration to
continue, but without the benefits of multipath.
Prefer ID_SERIAL over ID_SERIAL_SHORT (#1138254)

While there are devices that export multiple block devices with the same
ID_SERIAL_SHORT (e.g. card readers) they use different ID_SERIAL values for
those block devices. Using ID_SERIAL_SHORT instead of ID_SERIAL results in all
the card reader's block devices being considered "obvious multipath devices"
where multipathd lost the race condition game and didn't assemble them
correctly. Which is terribly wrong.