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
Update README.md
Remove _get_listening_port()

This was only used by the systemd path, which doesn't print
"Listening on http://...:$PORT" anymore.
Add ability to listen on different interfaces

This adds code to parse listener lines using the same syntax supported
by nginx[1]. This allows specifying ipv6 without having a dedicated
'ipv6' flag in the configuration file.

Parsing is very bare bones and not much checking is performed: only
basic tokenization is done and validation is performed by
getaddrinfo().

[1] http://nginx.org/en/docs/http/ngx_http_core_module.html#listen
lwan_status_*() formatting strings do not require '\n'

Newlines are added automatically.
Ensure _parse_listener() does not write on a read-only string