Have leader keep already-used invocation items

The other suggestion from Pat about potential extensions to bda0b3b.  I
think it's interesting from a story perspective, and hopefully nobody
will run into this by mistake when they hoped to use the Candelabra as a
light source later!  The specific language used by the leader could be
changed if someone has a better idea (as could the code itself, for that
matter).

src/dothrow.c

@@ -2033,7 +2033,30 @@ thitmonst(
 
         if (mon->mcanmove) {
             pline("%s catches %s.", Some_Monnam(mon), the(xname(obj)));
-            if (mon->mpeaceful) {
+            /* leader will keep tossed invocation item after you've done the
+               invocation and it's become unnecessary for completion.. */
+            if ((u.uevent.invoked && objects[obj->otyp].oc_unique
+                 && obj->otyp != AMULET_OF_YENDOR)
+                /* ...or any special item, if you've made him angry */
+                || !mon->mpeaceful) {
+                /* give an explanation for keeping the item only if leader is
+                   not doing it out of anger */ 
+                if (mon->mpeaceful && !Deaf) {
+                    /* just in case, identify the object so its name will
+                       appear in the message */
+                    fully_identify_obj(obj);
+                    verbalize("%s part in this is finished.",
+                              s_suffix(The(xname(obj))));
+                    verbalize(
+               "We will guard it in case it is ever needed again, %s forbid.",
+                              align_gname(u.ualignbase[A_ORIGINAL]));
+                }
+                if (*u.ushops || obj->unpaid) /* not very likely... */
+                    check_shop_obj(obj, mon->mx, mon->my, FALSE);
+                (void) mpickobj(mon, obj);
+            } else {
+                /* under normal circumstances, leader will say something and
+                   then return the item to the hero */ 
                 boolean next2u = monnear(mon, u.ux, u.uy);
 
                 finish_quest(obj); /* acknowledge quest completion */
@@ -2043,11 +2066,6 @@ thitmonst(
                     sho_obj_return_to_u(obj);
                 obj = addinv(obj); /* back into your inventory */
                 (void) encumber_msg();
-            } else {
-                /* angry leader caught it and isn't returning it */
-                if (*u.ushops || obj->unpaid) /* not very likely... */
-                    check_shop_obj(obj, mon->mx, mon->my, FALSE);
-                (void) mpickobj(mon, obj);
             }
             return 1; /* caller doesn't need to place it */
         }

src/quest.c

@@ -228,13 +228,6 @@ finish_quest(struct obj *obj) /* quest artifact or thrown unique item or faux
 {
     struct obj *otmp;
 
-    /*
-     * Possible extensions [would mostly need to be done in thitmonst()]:
-     *   if the invocation has already been performed, leader keeps any
-     *     thrown (or kicked) invocation item, perhaps stating the intent
-     *     to guard it for the future;
-     */
-
     if (obj && !is_quest_artifact(obj)) {
         /* tossed an invocation item (or [fake] AoY) at the quest leader */
         if (Deaf)