Add config option for IPv6

Keep IPv4 as the default for now.
Add IPv6 support to _setup_socket_normally()

This uses getaddrinfo() passing in the appropriate hint (AF_INET or
AF_INET6) based on the config setting.
Allow systemd to return an IPv6 socket
No else blocks where the if returns
Support IPv6 in lwan_request_get_remote_address()

Switch to using inet_ntop() which supports both protocol versions and
uses a caller provided buffer to avoid thread-safety issues.
Remove unnecessary braces