os: change the type of permissions argument for Open etc. to uint32.

Besides being more correct, it protects against people accidentally
exchanging the permission and open mode arguments to Open.

R=rsc
CC=golang-dev
https://golang.org/cl/1904045

src/pkg/os/file_unix.go

@@ -25,7 +25,7 @@ const DevNull = "/dev/null"
 // Open opens the named file with specified flag (O_RDONLY etc.) and perm, (0666 etc.)
 // if applicable.  If successful, methods on the returned File can be used for I/O.
 // It returns the File and an Error, if any.
-func Open(name string, flag int, perm int) (file *File, err Error) {
+func Open(name string, flag int, perm uint32) (file *File, err Error) {
 	r, e := syscall.Open(name, flag|syscall.O_CLOEXEC, perm)
 	if e != 0 {
 		return nil, &PathError{"open", name, Errno(e)}