Implement constructing a control channel reset client as standalone function

Implement constructing a control channel reset client as standalone function

This implements creating a reset packet without needing to setup
a full control session.

Patch v2: fix unit test not working without further commits

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220427223419.241904-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24240.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>