'daemon: dbus' is incomplete

'daemon: dbus' was part of Ubuntu Core 15.04, it was only for system services and it was removed in series 16 in favor of 'interfaces'. As of now,

1. snapd allows specifying 'daemon: dbus' but when you do it generates an incomplete systemd service file:

/usr/lib/snapd/snapd[2329]: task.go:303: DEBUG: 2016-12-12T14:33:18-06:00 ERROR [start snap.test-hello-dbus.test-hello-dbusd-system.service] failed with exit status 1: Failed to start snap.test-hello-dbus.test-hello-dbusd-system.service: Unit snap.test-hello-dbus.test-hello-dbusd-system.service is not loaded properly: Invalid argument.#012See system logs and 'systemctl status snap.test-hello-dbus.test-hello-dbusd-system.service' for details.

2. snapcraft doesn't allow you to specify it (which is good since the implementation doesn't work correctly)

3. http://snapcraft.io/docs/snaps/metadata incorrectly lists it

4. https://github.com/snapcore/snapd/wiki/Snap-format also incorrectly lists it

I adjusted '4' just now. I'm not sure where to file a bug for '3'.

For '1', we could consider updating the 'dbus' interface in some manner to have snapd properly tie in with systemd. It already supports 'system' dbus names, generates a dbus bus policy file and adjusts security policy such that a daemon started without Type=dbus will work. It should therefore be possible to let the developer specify 'daemon: dbus' under certain conditions. This needs some design since the systemd service files allow only one BusName, but snapd supports a daemon to specify multiple slots for acquiring multiple dbus well-known names.