turn on protection against 'sudden death' from entropy failure

[Zooko Wilcox-O'Hearn]

In the next release of openssl, there will be a new option to turn on protection against 'sudden death' from entropy failure:

http://www.imperialviolet.org/2013/06/15/suddendeathentropy.html

To enable it, you have to invoke a function, for example, if you invoke EC_KEY_set_nonce_from_hash then after that your ECDSA signature operations will be protected.

This ticket is to change pyOpenSSL to call that function before it does DSA or ECDSA signature operations, if the function is present in the version of OpenSSL.