MEDIUM: ssl: improve crt-list format to support negation

Improve the crt-list file format to allow a rule to negate a certain SNI :

        <crtfile> [[!]<snifilter> ...]

This can be useful when a domain supports a wildcard but you don't want to
deliver the wildcard cert for certain specific domains.
diff --git a/include/types/ssl_sock.h b/include/types/ssl_sock.h
index 1ded15e..a0b2d79 100644
--- a/include/types/ssl_sock.h
+++ b/include/types/ssl_sock.h
@@ -28,6 +28,7 @@
 struct sni_ctx {
 	SSL_CTX *ctx;             /* context associated to the certificate */
 	int order;                /* load order for the certificate */
+	int neg;                  /* reject if match */
 	struct ebmb_node name;    /* node holding the servername value */
 };