mint-arena: Add game system call to only clip to entities

trap_ClipToEntities is the same as trap_Trace except it doesn't clip to world.
mint-arena: BotCheckBlocked, don't clip to world

BotCheckBlocked does a trace to see if there is an entity blocking the bot.
However, trap_Trace clips to the world and can cause entities to go unnoticed.

So specifically only clip to entities. The distance of trace (3 units) is short
enough we don't need to be concerned about it going through walls (hopefully...).

Remove checking for ENTITYNUM_WORLD because it won't be returned by
trap_ClipToEntities.

----

To see when the bug is occuring, do something like

trap_Trace(&trace, ms->origin, mins, maxs, end, ms->entitynum, MASK_PLAYERSOLID);
trap_ClipToEntities(&trace2, ms->origin, mins, maxs, end, ms->entitynum, MASK_PLAYERSOLID);

if ( trace.entityNum != trace2.entityNum && trace2.entityNum != ENTITYNUM_NONE ) {
	Com_Printf("Trace mismatch: (world) %2d %f -> (ent) %2d %f\n",
		trace.entityNum, trace.fraction, trace2.entityNum, trace2.fraction );
}

if (!trace.startsolid && trace.entityNum != ENTITYNUM_WORLD && trace.entityNum != ENTITYNUM_NONE)
// yada yada