Nerf stairdancing

[VeryAngryFelid]

I hope I don't need to list reasons why stairdancing is bad.
http://crawl.homedns.org/crawl#lobby nerfed it by making player stop ascending/descending if player got any damage while doing so. It is still playable, even hard combo like pure melee HO of Qazlal with 33% HP penalty, no ranged attacks SpEn of Kiku with 33% HP penalty. (yes, Sp is hard in that branch because the branch has ways to prevent kiting/luring/pillardancing also), Mu with 33% HP penalty and without ways to heal etc.) and much more enjoyable/fun. The only exception was done for damage from being poisoned so player can still use stairs without resting first. I think another exception should be done for Torment, Tomb 3 was extremely hard otherwise. To make game more fair there was another change - player can always retreat upstairs as first action after entering a new level. That really helps with "You enter a level and are adjacent to a pack of Killer Bees", for example.