Hook FCNPC functions instead of using custom Create/DestroyBoss functions

[WoutProvost]

The custom create and destroy functions are stupid and should be replaced with hooking the FCNPC functions. The reason I previously decided to use custom functions was because I had long 'constructor' functions that had the possibility to set every variable. Since those long constructors are gone now, there's not really a point to not make it easier and hook the FCNPC functions.

FAI_INVALID_BOSS_ID will be replaced by INVALID_PLAYER_ID and FAI_MAX_BOSSES will be replaced by MAX_PLAYERS.

I should also look into how gmx would affect this change, since NPCs don't disconnect and reconnect on gmx and FCNPC_Create then just returns the npc id. Trying a fs reload doesn't seem to work then.