Force 'def1' method when --redirect-gateway is done through service

Force 'def1' method when --redirect-gateway is done through service

The service deletes all added routes when the client process (openvpn)
exits, causing the re-instated default route to disappear.
Fix by rewriting "--redirect-gateway" to "--redirect-gateway def1" when
routes are set using interactive service.

Only the behaviour on Windows with intereactive service is affected.

Trac: #778

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1480466372-2396-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13307.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>