add license statement to checkincludes, and have it return an error code

#!/usr/bin/python3

# Copyright 2018 The Tor Project, Inc.  See LICENSE file for licensing info.

import fnmatch

import os

    err(

"""To change which includes are allowed in a C file, edit the {} files in its

enclosing directory.""".format(RULES_FNAME))

    sys.exit(1)