`Arbiter::is_running` can be used to check if the current even-loop is currently
running; which should also work after the system has stopped. `System::is_set`
was updated to reflect what it actually does, it tells if the event loop has
started, which alone can't tell if it has stopped.
Signed-off-by: Jonathas-Conceicao <jadoliveira@inf.ufpel.edu.br>
