local: fix setting of btime on directories on Windows

Before this change this would give errors like this

    failed to set metadata on directory: failed to set birth (creation) time: Access is denied.

This was caused by opening the directory in the wrong mode.

backend/local/setbtime_windows.go

@@ -4,7 +4,6 @@
 package local
 
 import (
-	"os"
 	"syscall"
 	"time"
 )
@@ -13,7 +12,13 @@ const haveSetBTime = true
 
 // setBTime sets the birth time of the file passed in
 func setBTime(name string, btime time.Time) (err error) {
-	h, err := syscall.Open(name, os.O_RDWR, 0755)
+	pathp, err := syscall.UTF16PtrFromString(name)
+	if err != nil {
+		return err
+	}
+	h, err := syscall.CreateFile(pathp,
+		syscall.FILE_WRITE_ATTRIBUTES, syscall.FILE_SHARE_WRITE, nil,
+		syscall.OPEN_EXISTING, syscall.FILE_FLAG_BACKUP_SEMANTICS, 0)
 	if err != nil {
 		return err
 	}