dco_freebsd: implement dco_get_peer_stats()

dco_freebsd: implement dco_get_peer_stats()

This is "fetch read/write statistics for a single peer", complementing
dco_get_peer_stats_multi() "... for all peers", and it is called in
--client mode, and (!) in p2mp mode to check if --inactive thresholds
are reached.

The FreeBSD DCO module has no "give me stats for a single peer" call, so
we just call dco_get_peer_stats_multi() to get all of them - and that
function is modified to handle p2p or p2mp mode by checking mode == CM_TOP.

(dco_linux does about the same in dco_get_peer*() -> ovpn_handle_peer(),
after a few iterations, except that it can query for "just one peer")

"--inactive" still does not work on FreeBSD, because the code in forward.c
looks at counters that are not set by FreeBSD DCO.

v2:
  on AUTH_FAIL, 'dco' struct is not initialized yet -> SIGSEGV crash,
  verify that dco_peer_id is >= 0 before calling dco_get_peer_stats_multi()

Github: OpenVPN/openvpn#898

Change-Id: I38a040a9bdcb44933d4ca538f746af5c61011d7c
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Ralf Lici <ralf@mandelbit.com>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1350
Message-Id: <20251109084130.11463-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg34273.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>