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/bytes/buffer_test.go

@@ -165,7 +165,7 @@ func TestBasicOperations(t *testing.T) {
 			t.Error("ReadByte unexpected eof")
 		}
 		if c != data[1] {
-			t.Error("ReadByte wrong value c=%v", c)
+			t.Errorf("ReadByte wrong value c=%v", c)
 		}
 		c, err = buf.ReadByte()
 		if err == nil {