MINOR: resolvers: shut the warning when "default" resolvers is implicit

Shut the connect() warning of resolvers_finalize_config() when the
configuration was not emitted manually.

This shuts the warning for the "default" resolvers which is created
automatically for the httpclient.

Must be backported in 2.6.
diff --git a/include/haproxy/resolvers-t.h b/include/haproxy/resolvers-t.h
index dc1381b..b727463 100644
--- a/include/haproxy/resolvers-t.h
+++ b/include/haproxy/resolvers-t.h
@@ -163,6 +163,7 @@
 	struct {
 		const char *file;           /* file where the section appears */
 		int         line;           /* line where the section appears */
+		int         implicit;       /* config was auto-generated and must be silent */
 	} conf;                             /* config information */
 };
 
diff --git a/src/resolvers.c b/src/resolvers.c
index bcfd878..1f894ab 100644
--- a/src/resolvers.c
+++ b/src/resolvers.c
@@ -2509,8 +2509,10 @@
 					continue;
 				}
 				if (connect(fd, (struct sockaddr*)&ns->dgram->conn.addr.to, get_addr_len(&ns->dgram->conn.addr.to)) == -1) {
-					ha_warning("resolvers '%s': can't connect socket for nameserver '%s'.\n",
-						 resolvers->id, ns->id);
+					if (!resolvers->conf.implicit) { /* emit a warning only if it was configured manually */
+						ha_warning("resolvers '%s': can't connect socket for nameserver '%s'.\n",
+						            resolvers->id, ns->id);
+					}
 					close(fd);
 					err_code |= ERR_WARN;
 					continue;
@@ -3682,6 +3684,9 @@
 	err_code |= resolvers_new(&curr_resolvers, "default", "<internal>", 0);
 	if (err_code & ERR_CODE)
 		goto err;
+
+	curr_resolvers->conf.implicit = 1;
+
 	err_code |= parse_resolve_conf(NULL, NULL);
 	if (err_code & ERR_CODE)
 		goto err;