middleware/file: transfer from does not make sense (#314)

Make it return an error when you use `transfer from` when you're
not a secondary.

Add tests as well.

Fixes #310

middleware/file/setup_test.go

@@ -26,6 +26,13 @@ func TestFileParse(t *testing.T) {
 		shouldErr      bool
 		expectedZones  Zones
 	}{
+		{
+			`file ` + zoneFileName1 + ` miek.nl {
+				transfer from 127.0.0.1
+			}`,
+			true,
+			Zones{},
+		},
 		{
 			`file`,
 			true,