socket: fix IPv6 availability detection

If IPv6 is loaded with disable=1, any IPv6 functionality is completely
disabled until the ipv6 module is reloaded. Do not assume IPv6 is available just
because the module is present. Fixes startup error:

Oct 27 20:58:02 cooker kernel: IPv6: Loaded, but administratively disabled, reboot required to enable
Oct 27 20:58:02 cooker kernel: systemd[1]: Set hostname to <cooker>.
Oct 27 20:58:02 cooker kernel: systemd[1]: Netlink failure for request 2: Operation not supported
Oct 27 20:58:02 cooker kernel: systemd[1]: Failed to configure loopback device: Operation not supported