'Iu' vs unknown container contents

An inventory of unpaid items where more than one was present would
show
|> bag's contents    N zorkmids
if any of the items were inside a container whose contents aren't
known.  But if there was only one item (so container must be owned
by hero) the 'Iu' output menu was skipped for pline and yielded
|> scroll of magic mapping   133 zorkmids
Force the menu display if the lone unpaid item is inside a container
whose contents are unknown.

I'm not sure whether a hero-owned container can have both unknown
contents and an unpaid item in normal play.  I managed it while
trying to fix a reported problem--except I can no longer find the
relevant report--where itemized shop billing also revealed unseen
container contents (for any number of items, not just 1).  That isn't
fixed yet, but I want to get the simpler 'Iu' part out of the way.

src/mkobj.c

@@ -1,4 +1,4 @@
-/* NetHack 3.6	mkobj.c	$NHDT-Date: 1513298759 2017/12/15 00:45:59 $  $NHDT-Branch: NetHack-3.6.0 $:$NHDT-Revision: 1.129 $ */
+/* NetHack 3.6	mkobj.c	$NHDT-Date: 1518053380 2018/02/08 01:29:40 $  $NHDT-Branch: NetHack-3.6.0 $:$NHDT-Revision: 1.130 $ */
 /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */
 /* NetHack may be freely redistributed.  See license for details. */
 
@@ -579,6 +579,23 @@ struct obj *otmp;
     }
 }
 
+/* is 'obj' inside a container whose contents aren't known?
+   if so, return the outermost container meeting that criterium */
+struct obj *
+unknwn_contnr_contents(obj)
+struct obj *obj;
+{
+    struct obj *result = 0, *parent;
+
+    while (obj->where == OBJ_CONTAINED) {
+        parent = obj->ocontainer;
+        if (!parent->cknown)
+            result = parent;
+        obj = parent;
+    }
+    return result;
+}
+
 /*
  * Create a dummy duplicate to put on shop bill.  The duplicate exists
  * only in the billobjs chain.  This function is used when a shop object