windows: respect permission bits on file opening

Chmod toggles the FILE_ATTRIBUTES_READONLY flag depending on the
permission bits. That's a bit odd but I guess some compromises were made
at some point and this is what was chosen to map to a Unix concept that
Windows doesn't really have in the same way. That's fine. However, the
logic used in Chmod was forgotten from Open, which then manifested
itself in various places, most recently, go modules' read-only behavior.

This corresonds with the syscall CL 202439.

Updates golang/go#35033

Change-Id: Id8e74c5205057a74a35eda213516780b79a2aed2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/202440
Run-TryBot: Jason A. Donenfeld <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>
Reviewed-by: Alex Brainman <[email protected]>

Author: zx2c4

windows/syscall_windows.go

@@ -410,7 +410,11 @@ func Open(path string, mode int, perm uint32) (fd Handle, err error) {
 	default:
 		createmode = OPEN_EXISTING
 	}
-	h, e := CreateFile(pathp, access, sharemode, sa, createmode, FILE_ATTRIBUTE_NORMAL, 0)
+	var attrs uint32 = FILE_ATTRIBUTE_NORMAL
+	if perm&S_IWRITE == 0 {
+		attrs = FILE_ATTRIBUTE_READONLY
+	}
+	h, e := CreateFile(pathp, access, sharemode, sa, createmode, attrs, 0)
 	return h, e
 }