Update to the checkpoints system in A Magician's Hike

I have made a minor update to the checkpoint system following a much appreciated comment by Neonesque. What has changed:

• A checkpoint is set the first time you enter a room instead of setting one every time you change room. A subtle difference that can save a lot of undoing for some players.
• There is now a second type of checkpoint for a spoiler-ish thing at the end of the game. It works the same than the other checkpoints except for only doing it under some special condition. (I'm actually quite proud of how simple and elegant that condition was implemented.)

This should not break your save file if you've already started playing, but on the other hand you will benefit from the update only if you start a new game...