Add support for register-dns through interactive service

Add support for register-dns through interactive service

The call to the service returns promptly after delegating the job to
a thread, before the task is completed. In the thread, "net stop dnscache",
"net start dnscache", "ipconfig /flushdns" and "ipconfig /register-dns"
are executed in that order.

Parallel execution of these commands is prevented by a lock that is
common to all connections started by the service.

Note: "net stop .." is used instead of "sc stop.." as the latter can
return before the service has fully stopped (in STOP_PENDING state),
causing the subsequent start to fail.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1457671646-4322-1-git-send-email-selva.nair@gmail.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11354
Signed-off-by: Gert Doering <gert@greenie.muc.de>