Supports map of maps

[masibw]

Hi team, this PR solve iovisor#297.

We have to use the table's file descriptor when using BPF_HASH_OF_MAPS or BPF_ARRAY_OF_MAPS. But now gobpf doesn't have the method. And also we have to pass the unsafe.Pointer of the file descriptor. So, I made the SetMap method.

This is a example.

parentTable := bpf.NewTable(m.TableId("parent_list"), m)
	innerTable := bpf.NewTable(m.TableId("inner_list"), m)
    fd := innerTable.Fd()
     if err := parentTable.SetMap(unsafe.Pointer(parentKey), fd); err != nil {
 	fmt.Fprintf(os.Stderr, "Failed to add data to parent_list: %s\n", err)
 	os.Exit(1)
 }

Please review this, thanks.