strings.Split: make the default to split all.

Change the signature of Split to have no count,
assuming a full split, and rename the existing
Split with a count to SplitN.
Do the same to package bytes.
Add a gofix module.

R=adg, dsymonds, alex.brainman, rsc
CC=golang-dev
https://golang.org/cl/4661051

src/pkg/runtime/debug/stack_test.go

@@ -35,7 +35,7 @@ func (t T) method() []byte {
 */
 func TestStack(t *testing.T) {
 	b := T(0).method()
-	lines := strings.Split(string(b), "\n", -1)
+	lines := strings.Split(string(b), "\n")
 	if len(lines) <= 6 {
 		t.Fatal("too few lines")
 	}