MINOR: dns: implement extra 'hold' timers.

This adds new "hold" timers : nx, refused, timeout, other. This timers
will be used to tell HAProxy to keep an erroneous response as valid for
the corresponding period. For now they're only configured, not enforced.
diff --git a/include/types/dns.h b/include/types/dns.h
index b339249..acff53a 100644
--- a/include/types/dns.h
+++ b/include/types/dns.h
@@ -148,6 +148,10 @@
 	} timeout;
 	struct {			/* time to hold current data when */
 		int valid;		/*   a response is valid */
+		int nx;                 /*   a response doesn't exist */
+		int timeout;            /*   no answer was delivered */
+		int refused;            /*   dns server refused to answer */
+		int other;              /*   other dns response errors */
 	} hold;
 	struct task *t;			/* timeout management */
 	struct list curr_resolution;	/* current running resolutions */