s4:rpc_server/dcesrv_auth.c - Fix a RPC issue in conjunction with Windows 2000

s4:rpc_server/dcesrv_auth.c - Fix a RPC issue in conjunction with Windows 2000

Windows 2000 does strictly request header signing on some requests also if the
server doesn't provide it. But there is a small trick (don't reset the actual
session info) to make these special RPC operations work without a full header
signing implementation.

This fixes for example the list of domain groups in local groups when displayed
sing the local user/group management tool.

And this should finally fix bug #7113.

The patch was inspired by another one by tridge and abartlet: http://gitweb.samba.org/samba.git/?p=tridge/samba.git;a=commitdiff;h=2dc19e2878371264606575d3fc09176776be7729