if_ovpn: deal with v4 mapped IPv6 addresses
Openvpn defaults to binding to IPv6 sockets (with
setsockopt(IPV6_V6ONLY=0)), which we didn't deal with.
That resulted in us trying to in6_selectsrc_addr() on a v4 mapped v6
address, which does not work.
Instead we translate the mapped address to v4 and treat it as an IPv4
address.
Sponsored by: Rubicon Communications, LLC ("Netgate")