Fix failure to respect default port of proxy URLs
Currently we require explicitly specifying the port when configuring a proxy server, which is seriously weird. I take the fact that nobody reported a bug until 2022 to indicate that almost nobody is using proxies. Whatever. Let's assume that if no port is provided, the default port for the protocol should be used instead.
This was originally reported as GProxyResolver bug, but nothing is actually wrong there. It's actually GProxyAddressEnumerator that gets tripped up by URLs returned by GProxyResolver without a default port. This breaks GSocketClient.
Fixing this requires exposing GUri's _default_scheme_port() function to GIO. I considered copy/pasting it since it's not very much code, but I figure the private call mechanism is probably not too expensive, and I don't like code duplication.
Fixes #2832 (closed)