Rename DECRYPT_KEY_ENABLED to TLS_AUTHENTICATED

Rename DECRYPT_KEY_ENABLED to TLS_AUTHENTICATED

The macro's name suggests that the key is enabled and being used. But
the macro actually something different but similar enough that the name
was probably right at some point.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20201023120259.29783-5-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21221.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>