Clean up tls_authentication_status and document it

Clean up tls_authentication_status and document it

The gain of the used optimisation approach of using a array with a
calculated index in favour of simple ifs is questionable with modern
compilers and the readability of the function suffers.

Also change the return type from simple int to an enum and add comments
and doxygen documentation.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20201023120259.29783-4-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21224.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>