agent: Work around json-glib bogosity re empty strings in arrays

https://bugzilla.gnome.org/show_bug.cgi?id=730803

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
Closes #614
agent: Backport bus type option to dbus-json1.

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
web: Use 'payload' instead of 'protocol' option.

Because that is the correct spelling for it.  Previously, everything
was using the dbus-json2 payload type by accident.  (This mostly
worked, but not entirely.)

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
Closes #616
web: Release client of the #service page.

This was an oversight.
web: Add cockpit.set_watched_client function.
web: Show modal dialog when single-server pages get disconnected.

Fixes #598
web: Don't reconnect D_Bus clients when acquired

This is not useful anymore since we now can let the user deal with it
in the "Disconnected" dialog.