dnsdist: Load an openssl configuration file, if any, during startup

[rgacogne]

Short description

This way dnsdist will load the default OpenSSL configuration, or a custom one specified via the OPENSSL_CONF environment variable.
It allows loading an engine or configuration various options supported by OpenSSL.
This requires OpenSSL >= 1.1.0.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[pieterlexis]

Should the #if perhaps be centralized and do a #define PDNS_OPENSSL_RECENT_ENOUGH but a better name?

[rgacogne]

Should the #if perhaps be centralized and do a #define PDNS_OPENSSL_RECENT_ENOUGH but a better name?

I don't really see what you mean by that, we have several checks referring to different versions of OpenSSL and LibreSSL, with only one of thel being used twice?

[omoerbeek]

Maybe, but not all version nummbers are the same, though I also could live with "libressl must be new enough" test.

Additionally, SSL_CTX_set_min_proto_version is available as well with libressl >= 2.7, but SSL_CTX_get0_privatekeyis not at all.

[omoerbeek]

Current version of PR compiles and links fine on OpenBSD.

[rgacogne]

Additionally, SSL_CTX_set_min_proto_version is available as well with libressl >= 2.7, but SSL_CTX_get0_privatekey is not at all.

I guess we could try to properly detect if these functions are present during configure, but it's a bit out-of-scope for this PR that I would like to be able to merge quickly.