-0 sends NULL separated file-list to key-handler

with this change `-0` is turned into a more generic switch which can be
used to send NULL-separated file-list to the key-handler as well.
this also means `-0` no longer implicitly enables `-o`

Closes: https://github.com/nsxiv/nsxiv/issues/140

commands.c

@@ -59,7 +59,7 @@ bool cg_quit(arg_t _)
 	if (options->to_stdout && markcnt > 0) {
 		for (i = 0; i < filecnt; i++) {
 			if (files[i].flags & FF_MARK)
-				printf("%s%c", files[i].name, options->stdout_separator);
+				printf("%s%c", files[i].name, options->using_null ? '\0' : '\n');
 		}
 	}
 	exit(EXIT_SUCCESS);