Enable dhcp on tap adapter using interactive service

Enable dhcp on tap adapter using interactive service

Currently, if dhcp on the TAP interface is disabled, OpenVPN
on Windows tries to enable it using netsh but that succeeds only when
run with admin privileges.

When interactive service is available, delegate this task to the
service.

Trac: #1111
Tested on Windows 7

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: <1538510474-27602-2-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17517.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>