Port removal, stop-on-nothing-to-do, and select when sleeping

The aligned goal here is to allow for removing ports, not busy-looping
when there's no ports but something to wait on, and allow for stopping
when nothing is remaining.  Accomplishing the latter requires all
of the former.

* 8sync/agenda.scm (<make-port-request>, make-port-remove-request)
  (port-remove-request port-remove-request?, stop-on-nothing-to-do):
  New variables.

  (update-agenda-from-select!): Sleep (via select, currently) when
  there's no ports in the queue but we still have things scheduled.

  (start-agenda): New comments