kdbus: add policy database implementation

kdbus: add policy database implementation

This patch adds the policy database implementation.

A policy database restricts the possibilities of connections to own,
see and talk to well-known names. It can be associated with a bus
(through a policy holder connection) or a custom endpoint.

By default, buses have an empty policy database that is augmented on
demand when a policy holder connection is instantiated.

Policies are set through KDBUS_CMD_HELLO (when creating a policy
holder connection), KDBUS_CMD_CONN_UPDATE (when updating a policy
holder connection), KDBUS_CMD_EP_MAKE (creating a custom endpoint)
or KDBUS_CMD_EP_UPDATE (updating a custom endpoint). In all cases,
the name and policy access information is stored in items of type
KDBUS_ITEM_NAME and KDBUS_ITEM_POLICY_ACCESS.

See kdbus.policy(7) for more details.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>