Interlevel Travel

From 79, 702. The shaft thing wasn't completely implemented as designed so far.

1. < are extremely rare: they are only used in branch ends, forbubbles without stairs and dead ends. Or in vaults.
2. > are much rarer than now: it is only used in bubbles and vaults without exits.
3. ^ are used much more often. This is why we're doing this: ^ give an interesting, single use option. Allowing for both secret and known ^ should be good for T&D skill, we can tweak the chances.

My old comments on implementing this:

I think the way to go is to add new feature types for known and unknown shaft traps; these would then be replaced by actual traps after the level is built. Known shaft traps should function as down stairs for connectivity purposes.

If the dungeon feature and trap conversion is in, what's left to do is for one, replace most occurences of ESCAPE_HATCH_DOWN with DNGN_KNOWN_SHAFT_TRAP in dungeon.cc, in particular if DNGN_KNOWN_SHAFT_TRAP == DNGN_ESCAPE_HATCH_DOWN+1. The DNGN_FLOORs in _fixup_stone_stairs should becom DNGN_KNOWN_SHAFT_TRAP (both! (dpeg)). Second, there are a number of hidden places where hatches are placed: All the calls to _place_specific_stair in _roguey_level, _bigger_room, _plan_main etc.

2393 wonders about extra upstairs on Lair:1. (Upstairs escape hatches are converted to branch exit stairs.) The way that you're taken to one specific stair case while several lead out is mostly an artefact of the way levels are generated, and could stand to be improved. D:27 and Zot:1 is a related issue (and hell stairs lately).