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/testing/testing.go

@@ -287,7 +287,7 @@ func parseCpuList() {
 	if len(*cpuListStr) == 0 {
 		cpuList = append(cpuList, runtime.GOMAXPROCS(-1))
 	} else {
-		for _, val := range strings.Split(*cpuListStr, ",", -1) {
+		for _, val := range strings.Split(*cpuListStr, ",") {
 			cpu, err := strconv.Atoi(val)
 			if err != nil || cpu <= 0 {
 				println("invalid value for -test.cpu")