Implement client side handling of AUTH_PENDING message

Implement client side handling of AUTH_PENDING message

This allows a client to extend the timeout of pull-request response
while waiting for the user to complete a pending authentication. A
timeout of 60s for a normal authentication might still works for a
simple 2FA (but still challenging). With a sophisticated (or overly
complicated) web based authentication 60s are quite short.

To avoid not detecting network problem in this phase, we use the
constant sending of PUSH_REQUEST/AUTH_PENDING as keepalive signal
and still timeout the session after the handshake window time.

patch v2: typo fixes, invert if for sscanf

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <20210125125628.30364-3-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21491.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>