Fix --multihome for IPv6 on 64bit BSD systems.

Fix --multihome for IPv6 on 64bit BSD systems.

The old code only worked if "struct openvpn*pktinfo" happened to use
the same structure packing as the CMSG_SPACE() / CMSG_LEN() macros
(which are part of the official API, see RFC 2292).

Get rid of "struct openvpn_*_pktinfo" definitions, replace them by
an opaque buffer sized large enough to fit IPv4 and IPv6 packet info
messages, as defined by CMSG_SPACE(sizeof(struct ...)).

On 32 bit platforms, the net result is the same.  On 64 bit platforms,
the new buffer is bigger than openvpn_pktinfo was, fixing an overflow
with ipi6_ifindex corruption on reception, and EINVAL on sendmsg().

The IPv4 related changes are only side effects of using the new buffer.

Fixes: FreeBSD 10.3/amd64, FreeBSD 9.3/sparc64, OpenBSD 6.0/amd64,
       NetBSD 7.0.1/i386.

Note: --multihome for IPv4 on NetBSD is still broken and non-fixable(!)
       as NetBSD lacks the necessary kernel code for the sendmsg() side.

Verified that "--multihome works as well as before" on FreeBSD 7.4/amd64,
       NetBSD 5.1/amd64, OpenBSD 4.9/i386, Linux/x86_64, Linux/i386,
       OpenSolaris 10 (--multihome needs -D_XPG4_2, see trac #750)

See also: ip(4), ip6(4), recv(2)

Trac #634, #327, #28

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20161009100929.46472-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12626.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>