Add detection of active VPN connections for MSI packages

Add detection of active VPN connections for MSI packages

This patch adds a simple up/down detection for each TAP interface found.
All "up" TAP interface IDs are listed as an `ACTIVETAPINTERFACES` MSI
property.

This allows MSI packages to test for active VPN connections on updates.
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20181121163653.4904-1-simon@rozman.si>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17942.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>