Refactor sending commands to interactive service

Refactor sending commands to interactive service

Move writing the message buffer to the interactive service pipe and
reading acknowledgement to a function.

A minor bug in open_tun where the ack data could be read even after
a communication error is fixed.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1538510474-27602-3-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17519.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>