Accept an IPv6 between "[...]" as an fcgi server hostname

Here are some examples:
    php_handler = <fcgi, [::1]:9000>
    fcgi_app_server = [::1]:9000

The square brackets syntax is already used in URLs (http://[::1]:8080/)
and allows to mix an IPv6 address and a port number.
yaws_cgi: Try IPv6 and IPv4 connections to the fcgi server

When opening a connection to the fcgi server, both IPv6 and IPv4 name
resolutions are tried (IPv6 is tried first).

This allows to use IPv6-only fcgi servers.