plugin/forward: cap default connect attempts (#8365)

Default to two connect attempts per configured upstream so fast failures cannot spin until the request deadline. Track whether max_connect_attempts was explicitly configured so zero still opts into the legacy unbounded behavior.

Fixes #7723

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

plugin/forward/setup.go

@@ -293,6 +293,7 @@ func parseBlock(c *caddy.Controller, f *Forward) error {
 			return err
 		}
 		f.maxConnectAttempts = uint32(n)
+		f.maxConnectAttemptsSet = true
 	case "health_check":
 		if !c.NextArg() {
 			return c.ArgErr()