Console user security policy

Console user security policy

* bus/config-parser.c:
(struct PolicyType): Add POLICY_CONSOLE
(struct Element.d.policy): s/gid_or_uid/gid_uid_or_at_console
(start_busconfig_child): Sets up console element when
<policy at_console=""> is encountered in a policy file
(append_rule_from_element): Convert console elements to console
rules.

* bus/policy.c:
(bus_policy_create_client_policy): Add console rules to the client
policy based on if the client is at the console
(bus_policy_append_console_rule): New function for adding a
console rule to a policy
(bus_policy_merge): Handle console rule merging

* dbus/dbus-sysdeps.h: Added the DBUS_CONSOLE_DIR constant
where we check for console user files

* dbus/dbus-sysdeps.c:
(_dbus_file_exists): New function which checks if the given
file exists
(_dbus_user_at_console): New function which does the system
specific process of checking if the user is at the console

* dbus/dbus-userdb.c:
(_dbus_is_console_user): New function converts a UID to user name
and then calls the system specific _dbus_user_at_console to
see if the user is at the console and therefor a console user