BUG/MEDIUM: ssl: Fix regression about certificates generation

BUG/MEDIUM: ssl: Fix regression about certificates generation

Since the commit f6b37c67 ["BUG/MEDIUM: ssl: in bind line, ssl-options after
'crt' are ignored."], the certificates generation is broken.

To generate a certificate, we retrieved the private key of the default
certificate using the SSL object. But since the commit f6b37c67, the SSL object
is created with a dummy certificate (initial_ctx).

So to fix the bug, we use directly the default certificate in the bind_conf
structure. We use SSL_CTX_get0_privatekey function to do so. Because this
function does not exist for OpenSSL < 1.0.2 and for LibreSSL, it has been added
in openssl-compat.h with the right #ifdef.