s4:pyrpc: add py_dcerpc_ndr_pointer_deref/wrap() infrastructure

s4:pyrpc: add py_dcerpc_ndr_pointer_deref/wrap() infrastructure

Some idl files use more than one layer of unique pointers. e.g.

    NTSTATUS lsa_GetUserName(
            [in,unique] [string,charset(UTF16)] uint16 *system_name,
            [in,out,ref] lsa_String **account_name,
            [in,out,unique] lsa_String **authority_name
            );

In order to specify *io.in.authority_name = NULL,
we need to wrap the pointer value (lsa_String or None)
into an base.ndr_pointer() object.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=7113
BUG: https://bugzilla.samba.org/show_bug.cgi?id=11892

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>