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>

pkg/bindfilter/bind_filter.go

@@ -11,7 +11,6 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
-	"syscall"
 	"unsafe"
 
 	"golang.org/x/sys/windows"
@@ -244,7 +243,7 @@ func getFinalPath(pth string) (string, error) {
 		}
 		buf = make([]uint16, n)
 	}
-	finalPath := syscall.UTF16ToString(buf)
+	finalPath := windows.UTF16ToString(buf)
 	// We got VOLUME_NAME_DOS, we need to strip away some leading slashes.
 	// Leave unchanged if we ended up requesting VOLUME_NAME_GUID
 	if len(finalPath) > 4 && finalPath[:4] == `\\?\` && flags == 0x0 {