ConsoleKit support for at_console policy settings

[Doug Goldstein]

Currently there's <policy at_console="true"> which is explicitly
dependent on pam_console to create /var/run/console/$user files to mark
who's at the console.

A possible better implementation might be to have D-Bus query ConsoleKit
for the currently active and local session to figure out whether to
match this policy or not.

The other work around is to have ConsoleKit manage /var/run/console/$user

Just an idea since the /var/run/console/$user method seems a bit hackish.