mbedtls: don't print unsupported ciphers in insecure cipher list

mbedtls: don't print unsupported ciphers in insecure cipher list

Commit 447997dd refactored the --show-ciphers code, but introduced a bug
in mbedtls builds where non-AEAD/CBC cipher were printed too.  Those are
however unsupported (as openvpn will tell you when you try to use them).

This fixes that bug.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1539242400-27614-1-git-send-email-steffan.karger@fox-it.com>
URL: https://sourceforge.net/p/openvpn/mailman/message/36438012/
Signed-off-by: Gert Doering <gert@greenie.muc.de>