Print usage information to stdout instead of stderr

It doesn't make much sense to print usage into to stderr. What's more is
that it prevents users from easily piping the output to other
programs(i.e. grep)

src/dunst.c

@@ -391,8 +391,7 @@ void usage(int exit_status)
 {
         fputs("usage:\n", stderr);
         char *us = cmdline_create_usage();
-        fputs(us, stderr);
-        fputs("\n", stderr);
+        puts(us);
         exit(exit_status);
 }