2015 Code Sprint: use ovl () in C++ files. * __dsearchn__.cc, balance.cc, colloc.cc, data.cc, debug.cc, dirfns.cc, eig.cc, file-io.cc, getgrent.cc, getpwent.cc, givens.cc, help.cc, hess.cc, mgorth.cc, quad.cc: Replace assignments to retval(?) with calls to ovl().

--- a/libinterp/corefcn/dirfns.cc	Sat Dec 12 21:34:06 2015 -0500
+++ b/libinterp/corefcn/dirfns.cc	Sun Dec 13 07:59:40 2015 -0800
@@ -123,7 +123,7 @@
 @end deftypefn")
 {
   int nargin = args.length ();
- 
+
   if (nargin > 1)
     print_usage ();