sub Twiggy::Server::start_listen {
    my ($self, $app) = @_;
    my @listen = @{$self->{listen} || [ ($self->{host} || '') . ":" . ($self->{port} || 0) ]};
    for my $listen (@listen) {
        push @{$self->{listen_guards}}, $self->_create_tcp_server($listen, $app);
    }
    # XXX drop privileges here
}