Allow creating unhidden traps in special levels

des.trap({ type = "rust", seen = true });

include/hack.h

@@ -519,6 +519,7 @@ enum bodypart_types {
 #define MKTRAP_NOFLAGS       0x0
 #define MKTRAP_MAZEFLAG      0x1 /* trap placed on coords as if in maze */
 #define MKTRAP_NOSPIDERONWEB 0x2 /* web will not generate a spider */
+#define MKTRAP_SEEN          0x4 /* trap is seen */
 
 #define MON_POLE_DIST 5 /* How far monsters can use pole-weapons */
 #define PET_MISSILE_RANGE2 36 /* Square of distance within which pets shoot */

include/sp_lev.h

@@ -128,6 +128,7 @@ typedef struct {
     packed_coord coord;
     xchar x, y, type;
     boolean spider_on_web;
+    boolean seen;
 } spltrap;
 
 typedef struct {