replace some weight-related magic numbers

adds a header file include/nhconst.h  (I'm open to a better name)

src/trap.c

@@ -5366,7 +5366,7 @@ try_disarm(
     /* duplicate tight-space checks from test_move */
     if (u.dx && u.dy && bad_rock(gy.youmonst.data, u.ux, ttmp->ty)
         && bad_rock(gy.youmonst.data, ttmp->tx, u.uy)) {
-        if ((gi.invent && (inv_weight() + weight_cap() > 600))
+        if ((gi.invent && (inv_weight() + weight_cap() > WT_TOOMUCH_DIAGONAL))
             || bigmonst(gy.youmonst.data)) {
             /* don't allow untrap if they can't get thru to it */
             You("are unable to reach the %s!", trapname(ttype, FALSE));