openssl: only set RSA keys if required to do so

openssl: only set RSA keys if required to do so

Modern OpenSSL and LibreSSL both do not require SSL_CTX_set_tmp_rsa to
be called unless SSL_CTX_need_tmp_rsa returns true, and LibreSSL and
OpenSSL compiled with deprecation warnings both will fail if this
happens. This commit resolves that.

With this change, it becomes possible to use LibreSSL in place of
OpenSSL with Factor.