core: update dbus policy file

core: update dbus policy file

This patch does four things:

1. Adds more comments that clarify the order in which things appear in
   the file

2. All entries are placed in the order in which their SD_BUS_METHOD()
   macros appear in the C vtables.

3. A couple of missing entries are added that should be open to all or
   do polkit

4. Corrects the interface name for the GetProcesses() calls. They belong
   to the per-unit interface, not to Unit