VLAN: implement support for forwarding only pre-tagged VLAN packets

VLAN: implement support for forwarding only pre-tagged VLAN packets

By building on top of the VLAN basic support, allow the user to configure
the server in VLAN_TAGGED-only mode. This way, only packets that reach
the TAP interface with an 802.1Q header are considered for forwarding -
untagged packets are all dropped.

A VLAN-tagged packet is then treated like any other packet by the
OpenVPN routing engine, with the exception of being allowed to reach
only clients configured with the same VID.

The logic applies to all server-to-client and client-to-client traffic.

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: <20191009143422.9419-7-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18918.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>