[multipath] remove dead maps

[multipath] remove dead maps

A map is considered dead, and thus removed from DM, when all its
constituent paths have disappeared from the system (ie sysfs).

When all paths fail, we keep the map, hoping some path can come back
up.

Flush will fail if the map is claimed (mount, ...), notify that.

This policy is rather safe, because :
- multipath is exec'ed only upon "add path events", so maps can magically
  disappear
- claimed maps cannot disappear

This map purge is clearly an interactive maintenance operation.