refactor: allow default ports to be listened on by other protocols

Previously, using a default port for another protocol would result in an
error like the following
```
error: Invalid value "localhost:8080" for '--pg-listen <[HOST][:PORT]>': PGPROTO cannot listen on port 8080 because it is default port for HTTP
```

This commit allows default ports to be used by other protocols,
as long as the port is not already occupied.

Close #1254