Only Consider ExitLevel a Cheat When Used to Cheat

!2098 (merged) made exitlevel a cheat no matter what in co-op, which doesn't make a lot of sense considering #1104 (closed). This merge request fixes that by making exitlevel only considered a cheat under certain circumstances.

Those circumstances are:

• While in single player, consistent with the map command.
• While in a special stage, as you gain nothing from exiting a special stage early. It's basically the same as failing one.
• During the credits, since apparently you can use the command during them for some reason.
• If at least one player in multiplayer has already beaten the level, which is consistent with setting playersforexit to one.
• If the level being exited to is a level that has not been unlocked yet, consistent with the map command.

Unfortunately, this does not solve #1104 (closed) entirely as record attack emblems are still cheese-able by exiting a level you've already beaten in multiplayer. I personally see this as outside the scope of this merge request, and will be handling it separately.

Test exe: exitlevelfix.exe