MINOR: set IP_FREEBIND on IPv6 sockets in transparent mode

Lets set IP_FREEBIND on IPv6 sockets as well, this works since Linux 3.3
and doesn't require CAP_NET_ADMIN privileges (IPV6_TRANSPARENT does).

This allows unprivileged users to bind to non-local IPv6 addresses, which
can be useful when setting up the listening sockets or when connecting
to backend servers with a specific, non-local source IPv6 address (at that
point we usually dropped root privileges already).