Fixes T11587.

In Q473, a user had lighttpd configured with a nonstandard port (8080), and their configuration did not work because we omitted the port from the Host: header.

There is a good argument that our behavior was in the wrong, and D16464 fixed this, by adding the port to the Host: header.

However, in T11587, this broke a different user who had a configuration which did not expect a port number in a port 80 request.

Use a behavior which is more similar to what browsers do:

• If the protocol is http and the port is 80, or the protocol is https and the port is 443, omit the port.
• Otherwise, include the port.

I think there's a good argument that our current behavior is spec-compliant and servers that do not treat Host: domain.com:80 the same as Host: domain.com are in the wrong, but realistically the lifetime support cost of fighting this battle is almost certainly higher than following the "empirical spec" and doing what browsers do.

• Added some debugging code to dump the strings.
• Ran arc list to test non-signed requests.
• Ran bin/repository reparse --message ... on a cluster device to test signed requests.
• In both cases, saw proper outputs (for Host: domain.com, domain.com:123; for signing: domain.com:80, domain.com:123).