DCO: support key rotation notifications

DCO: support key rotation notifications

Allow the kernel driver to notify us that it's time to renegotiate keys.
The intent is to avoid IV re-use after 2^32 packets.

This is a first draft intended for discussion. The accompanying kernel
change for FreeBSD can be found in https://reviews.freebsd.org/D39570

Signed-off-by: Kristof Provost <kprovost@netgate.com>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20230414094227.9153-1-kprovost@netgate.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26590.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>