logging: Fix default access logger

[francislavoie]

Fix #6248

The problem is that we were skipping hosts that have "" as their logger name (which happens when using the log directive with no options).

This is a regression from #6088, but the reporter in #6248 had a different problem on 2.7.6 which I think we've since fixed (causing skip_hosts to not work).

Also, I added a SplitHostPort in a couple spots cause I realized during testing that we were using the host with the port when we shouldn't to look up the logger names. This more thoroughly completes the idea from #5881

[francislavoie]

I made an additional change to simplify the logic for reading the logger names config, removes the "retry without port" because really there should never be any config with a port, it makes it ambiguous.

I've also added a check in the HTTP app's Validate() to make sure there's no ports in the config (only relevant for hand-crafted JSON config, Caddyfile produces the right thing already). Technically a small breaking change, but it should ensure users fix their config so it works as expected.

[mholt]

Thanks Francis!