os: New Open API.

We replace the current Open with:
OpenFile(name, flag, perm) // same as old Open
Open(name) // same as old Open(name, O_RDONLY, 0)
Create(name) // same as old Open(name, O_RDWR|O_TRUNC|O_CREAT, 0666)

This CL includes a gofix module and full code updates: all.bash passes.
(There may be a few comments I missed.)

The interesting packages are:
        gofix
        os
Everything else is automatically generated except for hand tweaks to:
        src/pkg/io/ioutil/ioutil.go
        src/pkg/io/ioutil/tempfile.go
        src/pkg/crypto/tls/generate_cert.go
        src/cmd/goyacc/goyacc.go
        src/cmd/goyacc/units.y

R=golang-dev, bradfitzwork, rsc, r2
CC=golang-dev
https://golang.org/cl/4357052

generate_cert.go

@@ -50,7 +50,7 @@ func main() {
 		return
 	}
 
-	certOut, err := os.Open("cert.pem", os.O_WRONLY|os.O_CREAT, 0644)
+	certOut, err := os.Create("cert.pem")
 	if err != nil {
 		log.Fatalf("failed to open cert.pem for writing: %s", err)
 		return
@@ -59,7 +59,7 @@ func main() {
 	certOut.Close()
 	log.Print("written cert.pem\n")
 
-	keyOut, err := os.Open("key.pem", os.O_WRONLY|os.O_CREAT, 0600)
+	keyOut, err := os.OpenFile("key.pem", os.O_WRONLY|os.O_CREAT, 0600)
 	if err != nil {
 		log.Print("failed to open key.pem for writing:", err)
 		return