New interface to function 'luaL_openselectedlibs'

Instead of preloading all non-loaded libraries, there is another
mask to select which libraries to preload.

lua.c

@@ -618,7 +618,7 @@ static void doREPL (lua_State *L) {
 /* }================================================================== */
 
 #if !defined(luai_openlibs)
-#define luai_openlibs(L)	luaL_openlibs(L)
+#define luai_openlibs(L)	luaL_openselectedlibs(L, ~0, 0)
 #endif