[multipathd] more flexible remove_map()

[multipathd] more flexible remove_map()

Add two params to remove_map() :

- a function pointer to a waiter thread stop method
  NULL if you don't want to stop it (or know it is already down)
- an int purge_vec, to control the multipath vector maintenance

Also remove a superfluous block in stop_waiter_thread()

Mostly from the Edward Goggin, EMC, patch-chest.