Doomsday Engine

Various changes working towards a common, game-side mobj spawn supersystem.

• Commonized P_SpawnMapThing, moved to p_start.c and renamed spawnMapThing.
• Relocated translation logic applied to spawn spots for player starts five through eight and non-polyobj type angles to P_LoadMapObjs for now (this translation should actually be moved into dpWADMapLoader but due to inadequate domain-knowledge, this is not yet possible).
• Changed (jHeretic/jHexen): Unknown thing types in maps no longer result in a fatal error. Instead, they are ignored, a warning message is output to the console and map loading continues as normal.