librpc/ndr: add ndr_pull_append/pop()

librpc/ndr: add ndr_pull_append/pop()

They can be used to parse a fragmented NDR byte stream.

ndr_pull_append() appends more data that can be processed
and ndr_pull_pop() removed already processed data.

This will be used to implement dcerpc pipes, where we can get
a verify large amount of pipe chunks, once we processed a chunk
we can forget about the related data, but we may need to keep some
bytes in order to get the alignment right.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>