3.6.1: disable hardcoded SetRestartPoint

[ivan-mogilko]

Resolve #1306.

The purpose of this change is to remove a hardcoded call to SetRestartPoint at the start of the game, which may be unwanted if the game uses its own save rules or custom saves.

Users should be advised to call SetRestartPoint manually at the end of game_start() instead: that will have equivalent result.
This change needs to be added to every template, and also noted in the manual.

1. Disables hardcoded SetRestartPoint at the game start in the engine.
   NOTE: pre-3.6.1 games still have it run, for backwards compatibility.
2. On project upgrade, insert a SetRestartPoint() call to the game_start() found in GlobalScript.asc. This is for user's convenience. Ironically this is 99% of the changes in this PR. But I may disable or remove this if it's found to be not a good addition.

NOTE 2: for ags4 we should perhaps simply deprecate SetRestartPoint and remove it from use, but that's another problem.