polkit: Tweak policy to work for Cockpit.

By installing our own rules.
daemon: Run usermod via pkexec.

So that admins can change roles and password hashes of user accounts.
daemon: Perform systemd actions via pkexec and systemctl.

Instead of via D-Bus methods.  This enables admins to start/stop/etc
services.
test: Run more tests as "admin".

Fixes #1203
Fixes #1204
Fixes #1206