tls: Use auto DH selection with OpenSSL 1.1.0 and later

[notroj]

The current code only uses auto DH parameter selection with OpenSSL 3.0+

SSL_CTX_set_dh_auto was introduced in OpenSSL 1.1.0 in this commit openssl/openssl@09599b5

[dilyanpalauzov]

This needs adjustment:

cyrus-imapd/lib/imapoptions

Line 3142 in fdf228c

tls_server_cert. Used by OpenSSL before version 3.0. */

[notroj]

Thanks @dilyanpalauzov - updated

[notroj]

One other note: the approach taken in the existing code is to ignore any manually configured DH parameters for OpenSSL versions which support SSL_CTX_set_dh_auto.

It would be possible to change the logic to:
a) use manually configured params if present, or else
b) fall back to the automatic logic otherwise.
but it'd be more complex change.

[notroj]

Rebased to hopefully address the compiler warnings which were triggering CI failures.

[notroj]

Apparently too difficult for me to track all the places using that #define combination. I've adjusted it but if there is a preferred coding style here I can change that. Also would welcome any feedback on comment above before iterating further. #5156 (comment)

[dilyanpalauzov]

While here absolutely nothing depends on me, I would prefer if the only supported OpenSSL version by future versions of Cyrus IMAP (so 3.12 and beyond) is OpenSSL 3.0+, as upstream OpenSSL does not support lower versions.