[multipath] claimed path handling overall

[multipath] claimed path handling overall

Detecting path claims during the discovery, and all paths, exposes
us to open(..., O_EXCL) collisions. Paths could appear as claimed when
they are not actually.

Visible effect is, during a udev add storm (driver loading), rarely,
a map would not appear when it should have. This was reported by
Maxim Kozover.

This patch remove all pp->claimed usage and allows DM_DEVICE_CREATE to
fail when loading a map with claimed paths (with libdevmapper logging
shut). But cleanup the mess on error.