dco-linux: implement dco_get_peer_stats{, multi} API

dco-linux: implement dco_get_peer_stats{, multi} API

With this API it is possible to retrieve the stats for a specific peer
or for all peers and then update the userspace counters with the value
reported by DCO.

Change-Id: Ia3990b86b1be7ca844fb1674b39ce0d60528ccff
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230322192757.20767-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26481.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>