Cleanup print_details and add signature/ED certificate print

Cleanup print_details and add signature/ED certificate print

This commit cleans up the logic in the function a bit. It also makes it
more clear the the details printed in the second part of the message are
details about the peer certificate and not the TLS connection as such.
Also print the signature algorithm as this might help to identify
peer certificate that still use SHA1.

The new format with for TLS 1.3 and an EC certificate.

Control Channel: TLSv1.3, cipher TLSv1.3 TLS_AES_256_GCM_SHA384, peer
certificate: 384 bit EC, curve secp384r1, signature: ecdsa-with-SHA256

Using the more generic OpenSSL functions also allows use to correctly
print details about ED certificates:

Control Channel: TLSv1.3, cipher TLSv1.3 TLS_AES_256_GCM_SHA384, peer
certificate: 253 bit ED25519, signature: ED25519

Patch v2: Cleanup multiple calls to EVP_PKEY_id, minor code restructuring

Patch v3: Always initialise sig.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20210326175750.4772-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21861.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit ca57070630a1b2935ee606cc1309005b56eb925f)