Repair --multihome on FreeBSD for IPv4 sockets.

The code in link_socket_write_udp_posix_sendmsg() for the IP_RECVDESTADDR
case was sending a too-large control message (sizeof openvpn_pktinfo,
which is a union for IPv4+IPv6) instead of just openvpn_in4_pktinfo,
leading to sendmsg() refusing to send the packet.

Use RFC 2292 macros for alignment + size calculation.

Fix trac#327

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Lazy-Ack-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1390164697-1590-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8250
(cherry picked from commit 661d914c8732a208580b1eab167255c85da162c9)