Use accessors for DSA and RSA objects

[botovq]

In the upcoming LibreSSL version 3.5, the DSA and RSA structs will become opaque, so we should use the OpenSSL 1.1 setters and getters to fix the build. The relevant API has been available since LibreSSL 2.7. The last LibreSSL version (2.6) not having these accessors was EOL late 2018, so no users should be affected by this change.

[wtoorop]

Thanks. There is something to say for dropping support for libraries that are EOL yes, but we haven't done it that way before... I think I would prefer a more conservative approach... something like this?:

#if OPENSSL_VERSION_NUMBER < 0x10100000 || (defined(HAVE_LIBRESSL) &&  OPENSSL_VERSION_NUMBER < 0x20700000)

[botovq]

Thanks for your feedback, understood and PR amended. We need to use LIBRESSL_VERSION_NUMBER since OPENSSL_VERSION_NUMBER is a constant.

https://github.com/libressl-portable/openbsd/blob/bc1ae415b645733fb5b0b7a063efaba81beae65d/src/lib/libcrypto/opensslv.h#L6-L11

[wtoorop]

Perfect @botovq . Thanks!