Implement AUTH_FAIL, TEMP message support

Implement AUTH_FAIL, TEMP message support

This allows a server to indicate a temporary problem on the server and
allows the server to indicate how to proceed (i.e. move to the next server,
retry the same server, wait a certain time,...)

This adds options_utils.c/h to be able to unit test the new function.

Patch v2: Improve documentation, format man page better, comment that
          protocol-flags is not a user usable option.

Patch v3: cleanup parse_auth_failed_temp to use a simple const string
          instead of a buffer

Patch v4: move message + strlen(TEMP) to caller

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Heiko Hund <heiko@ist.eigentlich.net>
Message-Id: <20220914170134.2659433-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25210.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>