[session] move context manager logic in the hook controls context manager
The `init_hooks_mode_categories` and `reset_hooks_mode_categories` have no other
user that the context manager itself. Having dedicated public function is not
useful.
# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s specific hooks and operations"""