Add "classify" keyvalue to override monster classification.

src/dlls/headcrab.cpp

@@ -119,6 +119,11 @@ const char *CMHeadCrab::pBiteSounds[] =
 //=========================================================
 int	CMHeadCrab :: Classify ( void )
 {
+	if ( m_iClassifyOverride == -1 ) // helper
+		return CLASS_NONE;
+	else if ( m_iClassifyOverride > 0 )
+		return m_iClassifyOverride; // override
+	
 	return	CLASS_ALIEN_PREY;
 }