Add option to harden (EC)DSA nonces against weak RNGs.

Private key information is leaked by (EC)DSA signatures when nonces
are produced by a weak RNG. This CL adds an option (disabled by
default) to mix in the private key and the hash of message into the
nonces produced by RNG for (EC)DSA signatures. This mitigates the
weakness, provided the private key was generated by strong RNG.

Change-Id: I60dbf57bff3cfcdcbbeb18be5d9dfba523cc6bb8