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>

ea_test.go

@@ -6,9 +6,10 @@ package winio
 import (
 	"os"
 	"reflect"
-	"syscall"
 	"testing"
 	"unsafe"
+
+	"golang.org/x/sys/windows"
 )
 
 var (
@@ -82,7 +83,7 @@ func Test_SetFileEa(t *testing.T) {
 	}
 	defer os.Remove(f.Name())
 	defer f.Close()
-	ntdll := syscall.MustLoadDLL("ntdll.dll")
+	ntdll := windows.MustLoadDLL("ntdll.dll")
 	ntSetEaFile := ntdll.MustFindProc("NtSetEaFile")
 	var iosb [2]uintptr
 	r, _, _ := ntSetEaFile.Call(f.Fd(),