fix(multipathd.service): adapt to upstream multipath-tools unit file

fix(multipathd.service): adapt to upstream multipath-tools unit file

In the long run, it's desirable to be able to drop dracut's copy of
multipathd.service and use the upstream one from multipath-tools instead.
This patch makes a step in that direction.

With these changes, the only remaining difference is the support for
rd.multipath=0 and rd_NO_MULTIPATH, which must obviously be ignored in the
upstream unit.

The modifications in this patch are minor and will have no effect in the
initramfs.