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

src/pkg/exec/exec.go

@@ -49,7 +49,7 @@ func modeToFiles(mode, fd int) (*os.File, *os.File, os.Error) {
 		if fd == 0 {
 			rw = os.O_RDONLY
 		}
-		f, err := os.Open(os.DevNull, rw, 0)
+		f, err := os.OpenFile(os.DevNull, rw, 0)
 		return f, nil, err
 	case PassThrough:
 		switch fd {