Fix auth-token not being updated if auth-nocache is set

Fix auth-token not being updated if auth-nocache is set

This fixes the auth-token not being updated if auth-nocache is set. Our
set_auth_token method ensures that the auth-token always has a username
but is a little bit too strict in the check.

Also add doxygen documentation and remove null checks. We use this function
only with non-null pointers and it makes it a bit nicer to read.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20201130123928.21837-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21291.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit fb789947ab1eba3e68fb8e4b3551d095a53962bd)