add support for IPv6 servers

[fstern]

Hi!

I tried to use php-radius with an IPv6 only radius server, but I only get a "host not found" error.

rad_add_server() still uses AF_INET family, so only hosts with IPv4 adresses are supported.

[cmb69]

See https://bugs.php.net/bug.php?id=59619, which has a patch attached.