src: preserve IP route configuration on restart

During restart, openvpn does not provide the routing information to the
helper script. Make use of a new option to tell NetworkManager to re-use
the previous configuration.

The "preserve-routes" defines are obtained from the shared header file
"nm-vpn-plugin-macros.h" so that we don't have to bump required API.

https://bugzilla.redhat.com/show_bug.cgi?id=1231338
https://bugzilla.gnome.org/show_bug.cgi?id=750873