Live: let read API detect inactive streams, allow streams without fd

In live streaming, there are cases where a stream will be inactive from
beginning to end and so will never call bt_iter_next. For those cases,
we have to detect when we are reading an inactive stream which happens
when all other active streams have been closed.

We don't want to have the same checks in two places, so this fix removes
the return of EAGAIN in bt_iter_next and instead returns a NULL event
with the new BT_ITER_FLAG_RETRY flag set.

Also, handle cases where streams and traces have no associated file
descriptors.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>