Create basic infrastructure for IPv6 default gateway handling / redirection.

Create basic infrastructure for IPv6 default gateway handling / redirection.

- introduce get_default_gateway_ipv6() and add stub functions with the
  implementation plan to the 4 major code blocks here (Windows,
  Linux/Android, *BSD and Solaris, "others")

- add &rgi6 to print_default_gateway(), and teach it to print v4, v6
  or both, depending on the calling environment

- unlike IPv4 (today), get_default_gateway_ipv6() is passed the actual
  target IPv6 address of the server we're looking for, so we can handle
  more complicated routing setups ("default to eth0, vpn server to ppp0")
  correctly

- consequently, --show-gateway has an optional parameter now, the
  IPv6 address to look up (for debugging)

- document --show-gateway and the extra option in openvpn.8

Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1441985627-14822-5-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10087

Signed-off-by: Gert Doering <gert@greenie.muc.de>