allow coroutine.corunning to return the main thread

http://lua-users.org/lists/lua-l/2006-09/msg00751.html

stepmania/src/lua-5.1/src/lbaselib.c

@@ -585,10 +585,8 @@ static int luaB_costatus (lua_State *L) {
 
 
 static int luaB_corunning (lua_State *L) {
-  if (lua_pushthread(L))
-    return 0;  /* main thread is not a coroutine */
-  else
-    return 1;
+  lua_pushthread(L);
+  return 1;
 }