Make dragonhide affectable by rotting attacks

Dragon corpses are capable of rotting, so it's plausible that the
skin of a dead dragon might also be possible to rot.

This is unlikely to come up in actual play at present (unless
wishing for rotten DSM, it can only end up rotting as a consequence
of brown pudding attacks): the primary motivation is to open up new
possibilities for armor-damaging attacks, which in current NetHack
aren't very relevant in the late game because everyone is wearing
dragon scale mail.

doc/fixes3-7-0.txt

@@ -1563,6 +1563,7 @@ lycanthrope instincts keep hero from changing shape while next to hostiles
 scroll of enchant armor formula has changed (in particular, magical armor now
         gains less enchantment when enchanted than nonmagical armor and
         uncursed scrolls can sometimes enchant by more than one point)
+dragonhide can rot
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository

src/mkobj.c

@@ -2286,8 +2286,9 @@ is_rottable(struct obj *otmp)
 {
     int otyp = otmp->otyp;
 
-    return (boolean) (objects[otyp].oc_material <= WOOD
-                      && objects[otyp].oc_material != LIQUID);
+    return (boolean) ((objects[otyp].oc_material <= WOOD
+                       && objects[otyp].oc_material != LIQUID)
+                      || objects[otyp].oc_material == DRAGON_HIDE);
 }
 
 /*

src/objnam.c

@@ -1186,7 +1186,8 @@ add_erosion_words(struct obj *obj, char *prefix)
                          : is_corrodeable(obj) ? "corrodeproof "
                            : is_flammable(obj) ? "fireproof "
                              : is_crackable(obj) ? "tempered " /* hardened */
-                               : "");
+                               : is_rottable(obj) ? "rotproof "
+                                 : "");
 }
 
 /* used to prevent rust on items where rust makes no difference */