Remove tun-ipv6 Option. Instead assume that IPv6 is always supported.

Remove tun-ipv6 Option. Instead assume that IPv6 is always supported.

This option was useful when IPv6 tun support was non standard and was an
internal/user specified flag that tracked the Ipv6 capability of the tun
device.

All supported OS support IPv6. Also tun-ipv6 is pushable by the remote so
not putting tun-ipv6 does not forbid ipv6 addresses.

This commit also clean up a bit of the ipv6 related tun.c. Changes for
most platforms are minimal.

For linux a bit more cleanup is done:
- Remove compatibility defines that were added 2008
- Always use IFF_NO_PI for the linux tun and not only for IPv4 only tun
  setups (Android also always IFF_NO_PI works fine with Ipv6).

This commit also remove a non ipv6 fallback for tap driver from OpenVPN
2.2-beta or earlier and only warns.

Patch V2: Integrate Gert's comments
Patch V3: Remove tun_ipv4 option. It only used for MTU discovery and there
          it was wrong since it should on the transport protocol if at all
Patch V4: Completely remove support for NetBSD <= 4.0 and remove
          NETBSD_MULTI_AF defines
Patch V5: Assume generic OS in tun.c is also IPv6 capable. Add changes to
          man page. Fix typos/change message as suggest by David.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1476377656-3150-1-git-send-email-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12695.html
Signed-off-by: David Sommerseth <davids@openvpn.net>