Fix allowing/showing unsupported ciphers and digests

Fix allowing/showing unsupported ciphers and digests

This is a minimal version to hide the non-supported ciphers in these
show-cipher/show-digests listings. It also adds code to the kt_md_get/
kt_cipher_get functions to error out early instead of getting an ugly
backtrace with OpenSSL errors later when actually trying to use the
ciphers.

This allows make check to work again on with OpenSSL 3.0.

The changes are kept minimal to  avoid pulling in all the other refactoring
for OpenSSL 3.0.

This commit is partly cherry-picked from ab3f32b9.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220512121429.2096164-7-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24334.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>