Add simple reference-counted string allocator and use it for passing

around references to the sudoers path.  This lets us avoid making
copies of the sudoers path for the errorfile as well as each Defaults
entry.

plugins/sudoers/visudo_json.c

@@ -1030,8 +1030,8 @@ export_sudoers(const char *sudoers_path, const char *export_path,
 	if (!quiet)
 	    sudo_warnx(U_("failed to parse %s file, unknown error"), sudoers_path);
 	parse_error = true;
-	free(errorfile);
-	if ((errorfile = strdup(sudoers_path)) == NULL)
+	rcstr_delref(errorfile);
+	if ((errorfile = rcstr_dup(sudoers_path)) == NULL)
 	    sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
     }
     ret = !parse_error;