libndr: add support for unions to ndr_pull_struct_blob_noalloc() and ndr_push_struct_...

libndr: add support for unions to ndr_pull_struct_blob_noalloc() and ndr_push_struct_into_fixed_blob()

The union switch value is stored by ndr_token_store() which uses talloc to
manage ndr.switch_list. Preallocate a ndr_token array and ndr_token_list on the
stack of size ndr_token_list.fixed_alloc_count and optionally use that in
ndr_token_store().

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>