Allow shooting npcs

- Make `game::npc_at` return the npc object directly (not the index in `game::active_npc`)
- Add `game::critter_at` to get any kind of critter, be it a monster, a npc or the player, this should make many things easier, instead of
```
if(there a monster) {
    do something
} else if(there is a npc) {
    do something
} else if(there is the player) {
    do something
}
```
one can write
```
Creature *critter = critter_at();
if(critter != NULL) {
    do something
}
```
The function is currently only used in the projectile attack code, because the other code uses different functions for npc and monsters, those have not been ported to Creature.

Bug report here http://smf.cataclysmdda.com/index.php?topic=5324 should be fixed by this.