Add warning for the --show-groups command that some groups are missing

Add warning for the --show-groups command that some groups are missing

OpenSSL has a weird way of only reporting EC curves that are implemented
in a certain way in the list of all EC curves. Note this fact and point
out that also the very important curves X448 and X25519 are affected.

Change-Id: I86641bf60d62a50e9b2719e809d2429d65c00097
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20231009105714.34598-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg27193.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>