Reveal branch stairs only after traversing them

include/dungeon.h

@@ -36,6 +36,7 @@ typedef struct stairway { /* basic stairway identifier */
     d_level tolev;        /* where does it go */
     boolean up;           /* up or down? */
     boolean isladder;     /* ladder or stairway? */
+    boolean u_traversed;  /* hero has traversed this stair */
     struct stairway *next;
 } stairway;
 

include/patchlevel.h

@@ -17,7 +17,7 @@
  * Incrementing EDITLEVEL can be used to force invalidation of old bones
  * and save files.
  */
-#define EDITLEVEL 37
+#define EDITLEVEL 38
 
 /*
  * Development status possibilities.