dropping unpaid items vs containers (trunk only)

From a bug report, the 'D'
command would list 'u' as an item category choice if you were carrying
unpaid items inside a container, but if those were the only unpaid items
then nothing would happen once the dropping stage was reached.  Applied
to all menustyles (except partial, which bypasses categories and goes
straight to a menu listing all items).  There were two alternatives for
the fix:  suppress 'u' as an applicable category when it only applies
to container contents, or include the container among the drop candidates
even though it isn't an unpaid item itself.  I went with the latter; it's
simpler to implement and also feels a little more intuitive than behaving
like there aren't any unpaid items present.

src/invent.c

@@ -1233,7 +1233,7 @@ STATIC_PTR int
 ckunpaid(otmp)
 register struct obj *otmp;
 {
-	return((int)(otmp->unpaid));
+	return (otmp->unpaid || (Has_contents(otmp) && count_unpaid(otmp)));
 }
 
 boolean