MINOR: dns: give ability to dns_init_resolvers() to close a socket when requested

The function dns_init_resolvers() is used to initialize socket used to
send DNS queries.
This patch gives the function the ability to close a socket before
re-opening it.

[wt: this needs to be backported to 1.7 for next fix]
diff --git a/src/haproxy.c b/src/haproxy.c
index 4ad370d..6a8b5c2 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -1176,7 +1176,7 @@
 		exit(1);
 
 	/* initialize structures for name resolution */
-	if (!dns_init_resolvers())
+	if (!dns_init_resolvers(0))
 		exit(1);
 
 	free(err_msg);