MEDIUM: boringssl: support native multi-cert selection without bundling

This patch used boringssl's callback to analyse CLientHello before any
handshake to extract key signature capabilities.
Certificat with better signature (ECDSA before RSA) is choosed
transparenty, if client can support it. RSA and ECDSA certificates can
be declare in a row (without order). This makes it possible to set
different ssl and filter parameter with crt-list.
diff --git a/include/types/ssl_sock.h b/include/types/ssl_sock.h
index a384f05..e3a85ca 100644
--- a/include/types/ssl_sock.h
+++ b/include/types/ssl_sock.h
@@ -29,7 +29,8 @@
 struct sni_ctx {
 	SSL_CTX *ctx;             /* context associated to the certificate */
 	int order;                /* load order for the certificate */
-	int neg;                  /* reject if match */
+	uint8_t neg;              /* reject if match */
+	uint8_t key_sig;          /* TLSEXT_signature_[rsa,ecdsa,...] */
 	struct ssl_bind_conf *conf; /* ssl "bind" conf for the certificate */
 	struct ebmb_node name;    /* node holding the servername value */
 };