s4:rpc_server/lsa: implement the policy security descriptor

s4:rpc_server/lsa: implement the policy security descriptor

We now check the requested access mask in OpenPolicy*()
and return NT_STATUS_ACCESS_DENIED if the request is not granted.

E.g. validating a domain trust via the Windows gui requires this
in order prompt the user for the credentials. Otherwise
we fail any other call with ACCESS_DENIED later and the
gui just displays a strange error message.

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