MINOR: ssl: Factorize ssl_get_tmp_dh and append a cbk to its name

MINOR: ssl: Factorize ssl_get_tmp_dh and append a cbk to its name

In the upcoming OpenSSLv3 specific patches, we will make use of the
newly created ssl_get_tmp_dh that returns an EVP_PKEY containing DH
parameters of the same size as a bind line's RSA or DSA private key.
The previously named ssl_get_tmp_dh function was renamed
ssl_get_tmp_dh_cbk because it is only used as a callback passed to
OpenSSL through SSL_CTX_set_tmp_dh_callback calls.