Improve the behaviour of HeldOnceFor

It should only return true on the first time the handler has been held for the specified frames.

Reset level_on_revive_gain back to 1

Since it makes more sense according to its description.
Plus, currently it has some edge cases like when ml_revive_levels is 1.

Fix passing through the kill_player_flag gaurd on Unlimited mode

Fix settings for ML rewards and their probability not updating in-game

Fix kill_player_flag not being reset on revive from ML rewards

Make death natural when no revives are left

Fix limited mode not working if meta leveling isn't enabled

Fix integer settings returning float

Set initial level_on_revive_gain to 0

Add new respawn systems