Prevent impossible with secret doors in sokoban

Doesn't currently happen as no Soko levels use secret doors,
but handle them anyway gracefully.

Secret doors have their doormask set, but it's overloaded wall_info.
The display routines aren't happy with D_CLOSED in wall_info.

src/detect.c

@@ -1851,6 +1851,8 @@ sokoban_detect(void)
         for (y = 0; y < ROWNO; y++) {
             levl[x][y].seenv = SVALL;
             levl[x][y].waslit = TRUE;
+            if (levl[x][y].typ == SDOOR)
+                levl[x][y].wall_info = 0; /* see rm.h for explanation */
             map_background(x, y, 1);
             if ((obj = sobj_at(BOULDER, x, y)) != 0)
                 map_object(obj, 1);