cleaned up entity culling in server code, implemented new option - sv_cullentities_tr...

cleaned up entity culling in server code, implemented new option - sv_cullentities_trace - traces a line from eye to random locations in the model box, if a trace succeeds it marks the entity visibility timeout for 1 second in the future, and if it fails and the timeout has expired by then, it culls the entity.  this method works great for most cases, and is far more aggressive (and faster) than pvs culling

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@1496 d7cf8633-e32d-0410-b094-e92efae38249