Support validating Web domains that don't run on the standard HTTP(S) port(s)

Since I control the whole server in question, it would be pretty trivial to set up access on the standard HTTP(S) port(s) for the domain I'm trying to verify. However, I don't really want to do that, because I'd rather keep the standard port numbers open for other sites I'll move there in the future (served by a different daemon) without needing to then reverse proxy requests through the HTTPd.

Just a thought, since I'm trying to verify my ZNC domain (which other people do use), and can't because of this port issue.

The issue does not name a file, test, or entry point. Start by locating the domain-validation implementation and its tests, then determine how verification currently selects HTTP(S) ports; done means a domain served on nonstandard ports can be validated without regressing standard-port verification.

Written by the indexing model from the issue text.