core: consolidate all permissions checking into main D-Bus interface

core: consolidate all permissions checking into main D-Bus interface

Moves the system settings permissions checking into the core service's
permissions checking, which at the same time enables 3-way permission
reporting (yes, no, auth) instead of the old yes/no that we had for
system settings permissions before.  This allows UI to show a lock
icon or such when the user could authenticate to gain the permission.

It also moves the wifi-create permissions' namespace to the main
namespace (not .settings) since they really should be checked before
starting a shared wifi connection, rather than having anything to do
with the settings service.