Fixing 'docker save' on Windows.

Save was failing file integrity checksums due to bugs in both
Windows and Docker. This commit includes fixes to file time handling
in tarexport and system.chtimes that are necessary along with
the Windows platform fixes to correctly support save. With this
change, sysfile_backups for windowsfilter driver are no longer
needed, so that code is removed.

Signed-off-by: Stefan J. Wernli <swernli@microsoft.com>

system/chtimes_windows.go

@@ -0,0 +1,27 @@
+// +build windows
+
+package system
+
+import (
+	"syscall"
+	"time"
+)
+
+//setCTime will set the create time on a file. On Windows, this requires
+//calling SetFileTime and explicitly including the create time.
+func setCTime(path string, ctime time.Time) error {
+	ctimespec := syscall.NsecToTimespec(ctime.UnixNano())
+	pathp, e := syscall.UTF16PtrFromString(path)
+	if e != nil {
+		return e
+	}
+	h, e := syscall.CreateFile(pathp,
+		syscall.FILE_WRITE_ATTRIBUTES, syscall.FILE_SHARE_WRITE, nil,
+		syscall.OPEN_EXISTING, syscall.FILE_FLAG_BACKUP_SEMANTICS, 0)
+	if e != nil {
+		return e
+	}
+	defer syscall.Close(h)
+	c := syscall.NsecToFiletime(syscall.TimespecToNsec(ctimespec))
+	return syscall.SetFileTime(h, &c, nil, nil)
+}