fix github issue #742 - suit of no digestion

Reported by vultur-cadens:  slow digestion from wearing white dragon
scales/mail blocked per-turn hunger and didn't cause any hunger,
unlike ring of slow digestion which blocks per-turn hunger but still
causes some hunger as a worn ring itself.  If no rings or amulet
were worn, wearing the suit prevented the hero from ever burning any
nutrition.

Change to treat wearing the suit to be quite a bit like wearing a
ring, unless hero is also wearing an actual ring of slow digestion
(then the hunger cost of the suit is 'free').

Wearing a ring of slow digestion and another ring consumes two units
of nutrition every 20 turns, no matter what suit.  Wearing white
dragon scales/mail and two non-slow digestion rings now consumes three
units of nutrition every 20 turns.  Using the suit to effectively get
an extra ring finger isn't free.

Fixes #742

src/eat.c

@@ -3036,8 +3036,26 @@ gethungry(void)
          * +0 ring of protection might do something (enhanced "magical
          * cancellation") if hero doesn't have protection from some
          * other source (cloak or second ring).
+         *
+         * [If wearing duplicate rings whose effects don't stack,
+         * should they both consume nutrition, or just one of them?
+         * Two +0 rings of protection are treated as if only one,
+         * but this could apply to most rings.]
          */
         switch (accessorytime) { /* note: use even cases among 0..19 only */
+        case 0:
+            /* 3.7: if not wearing a ring of slow digestion, obtaining
+               that property from worn armor (white dragon scales/mail)
+               causes the armor to burn nutrition; since it's not
+               actually a ring, we don't check for it on the ring
+               turns; because of that, wearing two (non-slow digestion)
+               rings plus the armor consumes more nutrition that one
+               non-slow digestion ring plus ring of slow digestion */
+            if (Slow_digestion
+                && (!uright || uright->otyp != RIN_SLOW_DIGESTION)
+                && (!uleft || uleft->otyp != RIN_SLOW_DIGESTION))
+                u.uhunger--;
+            break;
         case 4:
             if (uleft && uleft->otyp != MEAT_RING
                 /* more hungry if +/- is nonzero or +/- doesn't apply or