- In case of default port setting, change it to `0` so that a random one
would be assigned for i2pd in case where user uncomments the setting
without changing the port
- In case of ipv6 setting change it to `true` since the default
behaviour is to not use IPv6, and the only reason to uncomment it
would be to set it to `true`.
- try to specify default values and/or default behaviour
- make things more consistent
- remove trailing whitespaces
- formatting