BasicManagement: Add a config flag for enabling

Remove the coupling to MediaRenderer: allow BasicManagement on any
rygel plugin, provided the configuration specifies that diagnostics
should be started.

Add a DIAGNOSTICS capability that. Also add empty X_DLNACAP element
to the media renderer description template so the new capability can
be added there if needed.

This change means that by default the service will not run. Add this to
rygel.conf to enable:
  [Playbin]
  diagnostics=true