Send stdout and stderr of OpenVPN started by interactive service to NUL

Send stdout and stderr of OpenVPN started by interactive service to NUL

Currently the service directs stdout/stderr of openvpn process to a pipe.
The
service never reads from it unless the process exits with an error. This
causes
the process to hang when large amount of log is written to stdout.

- Direct stdout/stderr to NUL
- Write the exit code (if nonzero) to the event log

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1455470881-32341-1-git-send-email-selva.nair@gmail.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11161
Signed-off-by: Gert Doering <gert@greenie.muc.de>