VLAN: add basic VLAN tagging support

VLAN: add basic VLAN tagging support

This patch introduces basic support for VLAN tagging on the server side.
The introduced functionality consists in allowing the user to assign
a VID to the server TAP device and a VID to each client port.

Client specific VID are assigned by means of files in CCD (like for
other client specific settings).

Once VIDs have been assigned, everything works as before, except that
communications are allowed only between hosts having the same VID.

With this patch all broadcast and client-to-client traffic is yet
separated by VLAN: only client-to-server unicasts are affected.

Signed-off-by: Fabian Knittel <fabian.knittel@lettink.de>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20191009143422.9419-3-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18924.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>