MUSE for branch stairs

MUSE for branch stairs

Fix another item in the "A few bugs" mail.  Monsters who wanted to flee
weren't able to use 'sstairs' (extra stairway leading to different branch
of dungeon) due to a logic error in the find_defensive() choices.
  if (terrain==STAIR) {
  } else if (terrain==LADDER) {
  } else if (x==sstairs.sx && y==sstairs.sy) {
  } else { /* check traps */
  }
wouldn't find 'sstairs' because they have terrain type STAIRS.  (Also,
the sstairs check wasn't screening out immobile monsters, but that bug
didn't have a chance to manifest.)

There's a bunch of reformatting, and some code re-organization to improve
other formatting, and some additional logic fixes.