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/exec/lp_unix.go

@@ -39,7 +39,7 @@ func LookPath(file string) (string, os.Error) {
 		return "", &Error{file, err}
 	}
 	pathenv := os.Getenv("PATH")
-	for _, dir := range strings.Split(pathenv, ":", -1) {
+	for _, dir := range strings.Split(pathenv, ":") {
 		if dir == "" {
 			// Unix shell semantics: path element "" means "."
 			dir = "."