route.c: improve get_default_gateway() logic on Windows

route.c: improve get_default_gateway() logic on Windows

When adding host route for IPv4, we use the default gateway. There are
cases, however, when this does not work - for example when remote
is not accessible via default gateway but via dedicated route.

Factor out code to look for the best gateway to reach the host from
get_default_gateway_ipv6() and generalize is for IPv4/6.

Change-Id: I6c7e1cef637fe9fb3f3bc6ff4fb2c65599cd86fb
Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250131154135.32169-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg30769.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>