Use stairs above or below when they exist

Sometimes when you went down stairs in close proximity, it could move
you to a different set of stairs - this is most visible in evac shelter
0 and 2, where going down the top left stairs could teleport you to the
  bottom right stairs. To fix this, if there are stairs below or above
the stairs you're using, go directly to those instead of searching the
map for stairs.

src/game.cpp

@@ -10226,10 +10226,24 @@ cata::optional<tripoint> game::find_or_make_stairs( map &mp, const int z_after,
     int best = INT_MAX;
     const int movez = z_after - get_levz();
     Creature *blocking_creature = nullptr;
+    const bool going_down_1 = movez == -1;
+    const bool going_up_1 = movez == 1;
+    // If there are stairs on the same x and y as we currently are, use those
+    if( going_down_1 && mp.has_flag( TFLAG_GOES_UP, u.pos() + tripoint_below ) ) {
+        stairs.emplace( u.pos() + tripoint_below );
+    }
+    if( going_up_1 && mp.has_flag( TFLAG_GOES_DOWN, u.pos() + tripoint_above ) ) {
+        stairs.emplace( u.pos() + tripoint_above );
+    }
+    if( stairs ) {
+        // We found stairs above or below, no need to do anything else
+        return stairs;
+    }
+    // Otherwise, search the map for them
     for( const tripoint &dest : m.points_in_rectangle( omtile_align_start, omtile_align_end ) ) {
         if( rl_dist( u.pos(), dest ) <= best &&
-            ( ( movez == -1 && mp.has_flag( flag_GOES_UP, dest ) ) ||
-              ( movez == 1 && ( mp.has_flag( flag_GOES_DOWN, dest ) ||
+            ( ( going_down_1 && mp.has_flag( TFLAG_GOES_UP, dest ) ) ||
+              ( going_up_1 && ( mp.has_flag( TFLAG_GOES_DOWN, dest ) ||
                                 mp.ter( dest ) == t_manhole_cover ) ) ||
               ( ( movez == 2 || movez == -2 ) && mp.ter( dest ) == t_elevator ) ) ) {
             if( mp.has_zlevels() && critter_at( dest ) ) {