MINOR: ssl: Create HASSL_DH wrapper structure

MINOR: ssl: Create HASSL_DH wrapper structure

The DH mechanism relies on DH objects that are low-level structures that
should not be used anymore starting from OpenSSLv3. With the newer
OpenSSL version, we should only use higher level EVP_PKEY objects.
Since enforcing this new logic to older versions of OpenSSL could be
dangerous (or plain impossible), we will keeptwo versions of the code
when required.
The HASSL_DH define will allow to unify some of the functions that were
created for DH use without having to add too many duplicated blocks of
code depending on the OpenSSL version.