FR: Mark stairs "known" only if both ends are pathable

[tedric]

The commit that prevented mapped stairs from being marked included a note that stairs seen through glass (or across water, or surrounded by plants, etc.) continue to be marked as "known" even though they may not be reachable, and that it "might not be ideal behavior". IMO it is definitely not ideal because accessing those areas is trivial but puts the burden on the player to remember to check all the up-stairs on the next level. Is it possible to have a staircase marked only when there's a viable path to it? Seems like there ought to be an easy way to implement that, since autotravel can identify the exact same thing.