MINOR: ssl: Add ssl_sock_get_dh_from_bio helper function

MINOR: ssl: Add ssl_sock_get_dh_from_bio helper function

This new function makes use of the new OpenSSLv3 APIs that should be
used to load DH parameters from a file (or a BIO in this case) and that
should replace the deprecated PEM_read_bio_DHparams function.
Note that this function returns an EVP_PKEY when using OpenSSLv3 since
they now advise against using low level structures such as DH ones.
This helper function is not used yet so this commit should be stricly
iso-functional, regardless of the OpenSSL version.