github issue #1060 - crystal helmet

Issue reported by vultur-cadens:  changing helm of brilliance to
crystal made it stop being classified as "hard helmet" so it gave
less protection against things falling onto the hero's head.

Change the is_metallic() tests used on helmets to new hard_helmet().
Unlike when thrown, crystal helmets don't break when objects fall
on them.

Fixes #1060

include/extern.h

@@ -564,6 +564,7 @@ extern int stop_donning(struct obj *);
 extern int Armor_off(void);
 extern int Armor_gone(void);
 extern int Helmet_off(void);
+extern boolean hard_helmet(struct obj *);
 extern void wielding_corpse(struct obj *, struct obj *, boolean);
 extern int Gloves_off(void);
 extern int Boots_on(void);