Fix server routes not working in topology subnet with --server [v3]

Fix server routes not working in topology subnet with --server [v3]

The IPv4 routing code needs an IPv4 address to point a route to, and
in --topology subnet mode, the *server* did not have one set by default.

So we now just default --route-gateway to the next address right after
the server address - the specific address doesn't matter, as the correct
next-hop will not be resolved by the host OS but by the OpenVPN daemon.
All that is needed is "it's in the subnet routed to the tun interface".

Using the server address itself would work on unix, but doesn't work with
the Windows TAP driver (as it does not spoof ARP responses for itself).

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1405254527-23833-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8904
(cherry picked from commit 4cc6a2595947a0e2f13b37637899bfc50f8509aa)