refine PR #946 - named armor

Move the new stuff from PR #946 out of xname() into new routine
armor_simple_name().

Noticed while testing:  tweak 'call object type' so that it doesn't
list instances of pre-discovered armor as likely candidates since
assigning a name to such wasn't showing up in the discoveries list.

Add "silver shield" as wishing synonym for "polished silver shield".

src/do_name.c

@@ -1489,6 +1489,9 @@ call_ok(struct obj *obj)
     if (!obj || !objtyp_is_callable(obj->otyp))
         return GETOBJ_EXCLUDE;
 
+    if (!interesting_to_discover(obj->otyp))
+        return GETOBJ_DOWNPLAY;
+
     return GETOBJ_SUGGEST;
 }