Add a source line to the header, because type must always be fallback

The dir list spec says that fallback files start with: /* type=fallback */ https://gitweb.torproject.org/torspec.git/tree/dir-list-spec.txt#n140

But in legacy/trac#24953 (moved), we change type to whitelist by default. This might cause some parsers to break.

Instead, we should add a new /* source=whitelist|fallback*/ line.