fix failure with function handles caused by changeset ea6997657614 * ov-fcn-handle.cc (octave_fcn_handle::set_fcn): Undo previous change. * ov-fcn.h (octave_function::src_file_name): New virtual function. * ov-builtin.h (octave_builtin::src_file_name): Rename from octave_builtin::fcn_file_name. * help.cc (do_which): Return source file name for built-in functions.