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>

hvsock_test.go

@@ -114,7 +114,7 @@ func TestHvSockListenerAddresses(t *testing.T) {
 
 	ra := rawHvsockAddr{}
 	sa := HvsockAddr{}
-	u.Must(socket.GetSockName(windows.Handle(l.sock.handle), &ra))
+	u.Must(socket.GetSockName(l.sock.handle, &ra))
 	sa.fromRaw(&ra)
 	u.Assert(sa == *addr, fmt.Sprintf("listener local addr give: %v; want: %v", sa, addr))
 }
@@ -162,7 +162,7 @@ func TestHvSockAddresses(t *testing.T) {
 			// {"server", sv.sock, _sla},
 		}
 		for _, tt := range localTests {
-			u.Must(socket.GetSockName(windows.Handle(tt.giveSock.handle), &ra))
+			u.Must(socket.GetSockName(tt.giveSock.handle, &ra))
 			sa.fromRaw(&ra)
 			if sa != tt.wantAddr {
 				t.Errorf("%s local addr give: %v; want: %v", tt.name, sa, tt.wantAddr)
@@ -177,7 +177,7 @@ func TestHvSockAddresses(t *testing.T) {
 			{"server", sv},
 		}
 		for _, tt := range remoteTests {
-			u.Must(socket.GetPeerName(windows.Handle(tt.giveConn.sock.handle), &ra))
+			u.Must(socket.GetPeerName(tt.giveConn.sock.handle, &ra))
 			sa.fromRaw(&ra)
 			if sa != tt.giveConn.remote {
 				t.Errorf("%s remote addr give: %v; want: %v", tt.name, sa, tt.giveConn.remote)