rsync: implement SSL/TLS, change URL

rsync: implement SSL/TLS, change URL

I've added rsync TLS termination to the xonotic.org reverse proxy and
updated its rsyncd config. Legacy Xonotic rsync updaters are still
supported.

These client scripts use openssl to verify the server certificate and
protect the rsync connection.

Changed the URL so the server config no longer needs to define a module
for every package, so rsync URLs can also be browsable via https, and so
we don't confusingly use beta.xonotic.org to update to a stable release.