Implement sending AUTH_PENDING challenges to clients

Implement sending AUTH_PENDING challenges to clients

This implements sending AUTH_PENDING and INFO_PRE messages to clients
that indicate that the clients should be continue authentication with
a second factor. This can currently be out of band (openurl) or a normal
challenge/response two like TOTP (CR_TEXT).

Unfortunately this patch spend so much time in review in openvpn2 that
the corosponding IV_SSO commit in openvpn3 (34a3f264) already made its
way to released products so changing this right now is difficult.

https://github.com/OpenVPN/openvpn3/commit/34a3f264f56bd050d9b26d2e7163f88a
f9a559e2

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20200519220004.25136-5-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19909.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>