Codechange: Convert saveload numbers to enum values.

(This was mostly achieved with a few in-place regexes)

src/settings.cpp

@@ -1377,7 +1377,7 @@ static void PrepareOldDiffCustom()
  */
 static void HandleOldDiffCustom(bool savegame)
 {
-	uint options_to_load = GAME_DIFFICULTY_NUM - ((savegame && IsSavegameVersionBefore(4)) ? 1 : 0);
+	uint options_to_load = GAME_DIFFICULTY_NUM - ((savegame && IsSavegameVersionBefore(SLV_4)) ? 1 : 0);
 
 	if (!savegame) {
 		/* If we did read to old_diff_custom, then at least one value must be non 0. */