Add "internal" to some bootstrap statuses when no exits are available.

If the consensus does not contain Exits, Tor will only build internal
circuits. In this case, relevant statuses will contain the word "internal"
as indicated in the Tor control-spec.txt. When bootstrap completes,
Tor will be ready to handle an application requesting an internal
circuit to hidden services at ".onion" addresses.

If a future consensus contains Exits, exit circuits may become available.

Tor already notifies the user at "notice" level if they have no exits in
the consensus, and can therefor only build internal paths.

Consequential change from #13718.