Fix dbus name owner monitoring

The NameOwnerChanged signal content is
* arg0 the dbus name that has changed ownership
* arg1 the private dbus name of the previous owner
* arg2 the private dbus name of the current owner

To catch loss of name owner, we want to receive signals
where arg0 is the name we are interested in and arg2 is
an empty string.

Also fixes memory leak in fake_owner_gone()