[multipathd] "add/del map" to keep the DM in sync

[multipathd] "add/del map" to keep the DM in sync

The reach that goal, the following changes were applied :

- call dm_lib_release() after each map creation/deletion
- coalesce_paths() new refwwid parameter to do scope-limiting
- flush_map() didn't really need the devname parameter, remove
- uev_{add,remove}_map extended to manipulate DM maps
- trap devmap umount events to try a little house-keeping
  (For testing. Might be removed latter)