fix issue #1309 - secret doors in Garden rooms

Issue reported by elunna:  when a room gets converted into a theme
room with fill type Garden, its walls are changed to trees but any
secret doors in those walls are still displayed as regular walls.

This adds a new D_ARBOREAL flag for secret doors, used to force them
to be displayed as a tree instead of a wall.

Fixes #1309

include/rm.h

@@ -219,6 +219,9 @@ struct rm {
 #define D_LOCKED  0x08
 
 #define D_TRAPPED 0x10
+/* secret doors aren't trapped or nonpasswall; overload D_TRAPPED and
+   W_NONPASSWALL for 'arboreal' secret doors (in garden theme room) */
+#define D_ARBOREAL D_TRAPPED
 #define D_SECRET  0x20 /* only used by sp_lev.c, NOT in rm-struct */
 
 /*