dco: Update counters when a client disconnects

dco: Update counters when a client disconnects

When the kernel module (Linux or FreeBSD) notifies us that a peer has
disconnected we'd like to get a final count of the in/out bytes for that
peer.
We can't request that information any more, because the kernel has
already removed the peer at that point.

Have the kernel send that information as part of the "delete peer"
notification, and update the counters a final time.

This implements the FreeBSD-specific DCO code, but not the
Linux-specific code. It will simply add 0 to the count on Linux.

Signed-off-by: Kristof Provost <kprovost@netgate.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221205164103.9190-3-kprovost@netgate.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25614.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>