MEDIUM: ssl: implement a workaround for the OpenSSL heartbleed attack

MEDIUM: ssl: implement a workaround for the OpenSSL heartbleed attack

Using the previous callback, it's trivial to block the heartbeat attack,
first we control the message length, then we emit an SSL error if it is
out of bounds. A special log is emitted, indicating that a heartbleed
attack was stopped so that they are not confused with other failures.

That way, haproxy can protect itself even when running on an unpatched
SSL stack. Tests performed with openssl-1.0.1c indicate a total success.