Manually marshall dom_sid, so we can use a fixed size array for

Manually marshall dom_sid, so we can use a fixed size array for
dom_sid.sub_auths rather than a dynamically allocated one.

This makes it possible to use the same DCE/RPC object code for Samba 3
and Samba 4's DCE/RPC parsers and allows copying sids more easily
(since they no longer contain any pointers). The cost of having additional
manual marshalling code is limited (~35 additional lines of C code).