Allow Authtoken lifetime to be short than renegotiation time

Allow Authtoken lifetime to be short than renegotiation time

Currently the life time of the auth-token is tied to the renegotiation
time.  While this is fine for many setups, some setups prefer a user
to be no longer authenticated when the user disconnects from the VPN
for a certain amount of time.

This commit allows to shorten the renewal time of the auth-token and
ensures that the server resends the auth-token often enough over the
existing control channel. This way of updating the auth token is a lot
more lightweight than the alternative (frequent renegotiations).

Patch v2: fix grammar mistakes (thanks Gert), fix unit tests

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221017095145.2580186-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25407.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>