VLAN: allow forwarding tagged and untagged packets on the server TAP device

VLAN: allow forwarding tagged and untagged packets on the server TAP device

This changes allows the user to configure the server TAP interface to
forward both VLAN tagged and untagged packets (i.e. vlan_accept ==
VLAN_ALL).

Untagged packets are marked with the VID configured in the server
configuration file, while tagged packets will keep their header as it
is.

Forwarding is then performed following the standard rules, while
ensuring that pakcets do not leave the VLAN they belong to.

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: <20191009143422.9419-8-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18919.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>