The diagnostic interface will now only come up when station is
connected. This avoids the need for display station to return
a 'connected' out parameter. We can instead just see that
the diagnostic interface doesn't exist.
The information requested with GetDiagnostics will now appear in
the "station <iface> show" command. If IWD is not connected, or
there is no diagnostic interface (older IWD version) 'show' will
behave as it always has, only showing scanning/connected.
Station, WSC, Ad-Hoc and AP family_arg_completion functions were
identical except for which commands they were referencing and the
interface type they were interested in. Combine all these into a single
function.
