Clean ups related to marking transporting serfs as LOST, and losing resources as a result

game: Split lose_transported_resource() into non-static game_cancel_transported_resource() and game_lose_resource().
flag: Remove obsolete flag_cancel_transported_stock().
serf: Move mark_serf_as_lost() into non-static serf_set_lost_state(), which should handle the switch from all states to LOST.
serf: Allow two serfs to change places independently of their individual states

Add function serf_is_waiting() and serf_switch_waiting(). The first checks if a serf is waiting to walk and in which direction; the second signals the serf to actually begin walking in the given direction by switching place with another serf. These functions make it possible for any state to switch with any other state.
Add parameter to get_walking_animation() to get special walking animation for switching with other serf.