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>

backup_test.go

@@ -6,7 +6,6 @@ package winio
 import (
 	"io"
 	"os"
-	"syscall"
 	"testing"
 
 	"golang.org/x/sys/windows"
@@ -202,7 +201,7 @@ func makeSparseFile() error {
 	}
 	defer f.Close()
 
-	err = syscall.DeviceIoControl(syscall.Handle(f.Fd()), windows.FSCTL_SET_SPARSE, nil, 0, nil, 0, nil, nil)
+	err = windows.DeviceIoControl(windows.Handle(f.Fd()), windows.FSCTL_SET_SPARSE, nil, 0, nil, 0, nil, nil)
 	if err != nil {
 		return err
 	}