Implement on-link route adding for iproute2

This is an OpenPGP/MIME signed message (RFC 4880 and 3156)
Currently, when compiling with --enable-iproute2 , OpenVPN does not
create a correct route when the user is connected to the Internet
without a gateway (e.g. via ppp). This patch implements the
corresponding FIXME.

Signed-off-by: Philipp Hagemeister <phihag@phihag.de>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <54259015.2030005@phihag.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9056
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit baa195b9884e276c4fd3dc0c9e8a84b89ea71cfb)