Very simple combat; will add much more later.
Add pack size and chance of appearing in a pack, to Animal_datum.
New enum, Cause_of_death - the reasons a citizen may die.  Lookup/name functions for Cause_of_death.
New function, Map_tile::choose_hunt_animal(), returns an Animal chosen from the animals on this tile (or ANIMAL_NULL if we failed to find one.
Use hunting skill level to modify the chances offinding nothing in Map_tile::choose_hunt_animal().
New Race_skill SKILL_LIVESTOCK, covers livestock care, husbandry, slaughtering and capturing (only the last is used at this point).
New Player_city functions: kill_citizens() kills citizens and takes care of employment (if we killed employed citizens).  get_employers() returns all buildings that have at least one worker of the given Citizen_type.  do_hunt() finds animals and attempts to kill or capture them.  kill_animals() collects resources from killing animals.  Use kill_citizens() when citizens die of starvation.