dssync: allow replications of a single obj with net rpc vampire keytab.

dssync: allow replications of a single obj with net rpc vampire keytab.

This is triggered by setting the new "single" flag in the dssync_context
and filling the "object_dn" member with the dn of the object to be
fetched.

This call is accomplished by specifying the DRSUAPI_EXOP_REPL_OBJ
extended operation in the DsGetNCCHanges request. This variant does
honor an up-to-date-ness vectore passed in, but the answer does not
return a new up-to-dateness vector.

Call this operation as "net rpc vampire keytab /path/keytab object_dn" .

Michael
(This used to be commit f4a01178a3d8d71f416a3b67ce6b872420f211c0)