Fix server side SSLEngine ServerKeyExchange signature.

Code using SSLEngine for non-blocking SSL can't talk with openssl as a client,
since the signature is computed on different content (and openssl checks it,
unlike java). The fix is to use strip the 0x00 prefix when signing - like
it is done when generating the message, refactored both to use a common
method. We also include the length in the signature, it was also missing.
2 files changed