MINOR: ssl: Add ssl_sock_set_tmp_dh_from_pkey helper function

MINOR: ssl: Add ssl_sock_set_tmp_dh_from_pkey helper function

This helper function will only be used with OpenSSLv3. It simply sets in
an SSL_CTX a set of DH parameters of the same size as a certificate's
private key. This logic is the same as the one used with older versions,
it simply relies on new APIs.
If no pkey can be found the SSL_CTX_set_dh_auto function wll be called,
making the SSL_CTX rely on DH parameters provided by OpenSSL in case of
DHE negotiation.