Allow spiders to spin webs

Revisited this feature, and the chance of a spider spinning a web
depends now on the number of webs already present on the level.

For a giant spider to spin a web in the middle of a room with no
supports, the limit of existing webs is 4, next to one support 9,
next to two supports 14, and so on. Cave spider limits are much lower.

src/trap.c

@@ -5399,6 +5399,22 @@ t_at(register int x, register int y)
     return (struct trap *) 0;
 }
 
+/* return number of traps of type ttyp on this level */
+int
+count_traps(int ttyp)
+{
+    int ret = 0;
+    struct trap *trap = g.ftrap;
+
+    while (trap) {
+        if (trap->ttyp == ttyp)
+            ret++;
+        trap = trap->ntrap;
+    }
+
+    return ret;
+}
+
 void
 deltrap(register struct trap* trap)
 {