Switch from sycall to windows (#295)

Where ever possible, use `golang.org/x/sys/windows` instead of `syscall`
(which has been deprecated since go1.11).

Using `windows.LocalFree` requires using `unsafe.Pointer`, which ensures
that the Go garbage collector does not try to free memory pre-maturely
if it was previously declared as a pointer.

Since `syscall.Handle` is part of API for `vhd` package, it was left
unchanged.

For security descriptor functions, switch to using
`windows.SECURITY_DESCRIPTOR` to avoid unnecessary byte manipulation and
panics due to missing input validation and error checking.

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

backuptar/tar.go

@@ -11,7 +11,6 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
-	"syscall"
 	"time"
 
 	"github.com/Microsoft/go-winio"
@@ -106,7 +105,7 @@ func BasicInfoHeader(name string, size int64, fileInfo *winio.FileBasicInfo) *ta
 	hdr.PAXRecords[hdrFileAttributes] = fmt.Sprintf("%d", fileInfo.FileAttributes)
 	hdr.PAXRecords[hdrCreationTime] = formatPAXTime(time.Unix(0, fileInfo.CreationTime.Nanoseconds()))
 
-	if (fileInfo.FileAttributes & syscall.FILE_ATTRIBUTE_DIRECTORY) != 0 {
+	if (fileInfo.FileAttributes & windows.FILE_ATTRIBUTE_DIRECTORY) != 0 {
 		hdr.Mode |= cISDIR
 		hdr.Size = 0
 		hdr.Typeflag = tar.TypeDir
@@ -396,7 +395,7 @@ func FileInfoFromHeader(hdr *tar.Header) (name string, size int64, fileInfo *win
 		fileInfo.FileAttributes = uint32(attr)
 	} else {
 		if hdr.Typeflag == tar.TypeDir {
-			fileInfo.FileAttributes |= syscall.FILE_ATTRIBUTE_DIRECTORY
+			fileInfo.FileAttributes |= windows.FILE_ATTRIBUTE_DIRECTORY
 		}
 	}
 	if creationTimeStr, ok := hdr.PAXRecords[hdrCreationTime]; ok {