Allow webs to be placed without a giant spider on them

Allow creating webs without spiders in the lua level scripts:

des.trap({ type = "web", spider_on_web = 0 });

Based on xNetHack commit by copperwater <aosdict@gmail.com>.

Also changes the Spider nest themed room to generate without
spiders when the level difficulty is 8 or less.

include/hack.h

@@ -475,6 +475,11 @@ enum bodypart_types {
 #define TELEDS_ALLOW_DRAG 1
 #define TELEDS_TELEPORT   2
 
+/* flags for mktrap() */
+#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 MON_POLE_DIST 5 /* How far monsters can use pole-weapons */
 #define PET_MISSILE_RANGE2 36 /* Square of distance within which pets shoot */