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.
diff --git a/include/proto/openssl-compat.h b/include/proto/openssl-compat.h
index ea92072..9b67109 100644
--- a/include/proto/openssl-compat.h
+++ b/include/proto/openssl-compat.h
@@ -89,6 +89,19 @@
 }
 #endif
 
+#if (OPENSSL_VERSION_NUMBER < 0x10002000L) || defined(LIBRESSL_VERSION_NUMBER)
+/*
+ * Functions introduced in OpenSSL 1.0.2 and not yet present in LibreSSL
+ */
+EVP_PKEY *SSL_CTX_get0_privatekey(const SSL_CTX *ctx)
+{
+	if (ctx->cert != NULL)
+		return ctx->cert->key->privatekey;
+	else
+		return NULL;
+}
+#endif
+
 #if (OPENSSL_VERSION_NUMBER < 0x1010000fL) || defined(LIBRESSL_VERSION_NUMBER)
 /*
  * Functions introduced in OpenSSL 1.1.0 and not yet present in LibreSSL