maint: Use "return ovl (...)" in DEFUN macros. * cellfun.cc, data.cc, debug.cc, defaults.cc, det.cc, dirfns.cc, dlmread.cc, error.cc, file-io.cc, getrusage.cc, graphics.cc, help.cc, input.cc, inv.cc, kron.cc, load-path.cc, pager.cc, pinv.cc, rand.cc, strfns.cc, urlwrite.cc, chol.cc, ov-classdef.cc, ov-struct.cc: Use "return ovl (...)" in DEFUN macros, rather than "return octave_value (...)".

--- a/libinterp/corefcn/strfns.cc	Thu Jan 21 11:24:36 2016 -0500
+++ b/libinterp/corefcn/strfns.cc	Thu Jan 21 09:47:50 2016 -0800
@@ -259,6 +259,7 @@
         }
     }
 
+  // Cannot use ovl.  Relies on overloaded octave_value call.
   return octave_value (result, '\'');
 }