[BUGFIX] Prevent basement-related teleportation by constraining stair searching to current  overmap tile

Tested a few mines / hotels / labs to ensure they don't break. In theory, stairs would be generated in the same 2x2 submap chunk everywhere.

Symptoms, requires stars to be aligned and/or mapshift border straddling or overmaptile-misaligned-map(tm): 

* North house with south stairs, basement with south stairs +  south house with north stairs + basement with north stairs: Magically transport from basement of 1 house to another.
* Go down + go up; you'll end up outside
* Go down, go to left stairs, go up, save; you'll reload outside.