Rewrite CSQC's setmodel a bit.

Rewrite CSQC's setmodel a bit.
Add support for CSQC_Ent_Spawn to csqc. A function that is called when a entity packet from the server arrives that requires a new entity.
entity( float entnum ) CSQC_Ent_Spawn;
CSQC_Ent_Spawn needs to spawn a new entity and set its .entnum field accordingly or simply return another existing entity or world if it wants to ignore that server entity. Subsequent CSQC_Ent_Update calls are routed to that entity (or CSQC_Ent_Spawn is called again when .Version increases).

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@8063 d7cf8633-e32d-0410-b094-e92efae38249