get_default_gateway_ipv6(): *BSD / MacOS / Solaris PF_ROUTE implementation

get_default_gateway_ipv6(): *BSD / MacOS / Solaris PF_ROUTE implementation

As for IPv4, a common implementation for all (supported) BSD families
and Solaris.  Supporting the latter requires separate implementations
for IPv4 and IPv6, unfortunately, so it's quite a bit of duplicate
code.

Further, extend add_route_ipv6() and delete_route_ipv6() to handle
link-local gateway addresses that require "gateway + interface" in
scoped notation ("fe80::1%em0").

Tested on FreeBSD 7.4/amd64, 9.3/sparc64, 10.0/amd64, NetBSD 5.1/amd64,
OpenBSD 4.9/i386, MacOS X 10.5/32 and 10.10/64 and OpenSolaris 10.11/i386.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1441985627-14822-9-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10082