logind: bring bus policy up-to-date

A while back we opened up all of logind's bus calls to unprivileged
users, via PK. However, the dbus1 policy wasn't updated accordingly.

With this change, the dbus1 policy is opened up for all bus calls that
should be available to unprivileged clients.

(also rearranges some calls in the vtable, to make more sense, and be in
line with the order in the bus policy file)

Fixes #471.