Allow dropping just picked up items

When using a menu to drop or put in items into a container,
allow putting in the item (or items) you picked up previously,
by selecting the 'P' entry from the item class menu

Inspired by the itemcat patch by Stanislav Traykov.

Invalidates saves and bones.

src/mkobj.c

@@ -400,6 +400,7 @@ splitobj(struct obj *obj, long num)
     otmp->quan = num;
     otmp->owt = weight(otmp); /* -= obj->owt ? */
     otmp->lua_ref_cnt = 0;
+    otmp->pickup_prev = 0;
 
     g.context.objsplit.parent_oid = obj->o_id;
     g.context.objsplit.child_oid = otmp->o_id;
@@ -765,6 +766,7 @@ mksobj(int otyp, boolean init, boolean artif)
     unknow_object(otmp); /* set up dknown and known: non-0 for some things */
     otmp->corpsenm = NON_PM;
     otmp->lua_ref_cnt = 0;
+    otmp->pickup_prev = 0;
 
     if (init) {
         switch (let) {