MEDIUM: ssl: Add options to forge SSL certificates

MEDIUM: ssl: Add options to forge SSL certificates

With this patch, it is possible to configure HAProxy to forge the SSL
certificate sent to a client using the SNI servername. We do it in the SNI
callback.

To enable this feature, you must pass following BIND options:

 * ca-sign-file <FILE> : This is the PEM file containing the CA certitifacte and
   the CA private key to create and sign server's certificates.

 * (optionally) ca-sign-pass <PASS>: This is the CA private key passphrase, if
   any.

 * generate-certificates: Enable the dynamic generation of certificates for a
   listener.

Because generating certificates is expensive, there is a LRU cache to store
them. Its size can be customized by setting the global parameter
'tune.ssl.ssl-ctx-cache-size'.