base: Allow cockpit.format() to accept variable argument lists

These are simpler than constructing an array of arguments.
shell: Switch to cockpit.format() for string formatting
shell: Migrate networking code to dbus-json3

This is the first step to moving the Networking page into a component.

Closes #1511
bridge: Remove support for dbus-json2

It's no longer in use now that the networking code has been
migrated to dbus-json3

We also no longer need the CockpitFakeManager code since all of
the dbus-json1 callers are interacting directly with ObjectManager
based services.

Fixes #1388