throughout: fix broken calls to Printf etc.

I have written a tool to verify Printf calls, and although it's not
ready to be reviewed yet it's already uncovered a spate of problems
in the repository.  I'm sending this CL to break the changes into
pieces; as the tool improves it will find more, I'm sure.

R=rsc
CC=golang-dev
https://golang.org/cl/3427043

src/pkg/xml/xml_test.go

@@ -301,7 +301,7 @@ func TestIssue569(t *testing.T) {
 	err := Unmarshal(buf, &i)
 
 	if err != nil || i.Field_a != "abcd" {
-		t.Fatalf("Expecting abcd")
+		t.Fatal("Expecting abcd")
 	}
 }