Add <port-remove-request> and friends

* 8sync/agenda.scm (<port-remove-request>, make-port-remove-request)
  (port-remove-request?, port-remove-request-port, port-remove-request)
  (%port-remove-request, agenda-handle-port-remove-request!): New variables
* 8sync/agenda.scm (%port-request): Original syntax of this probably
  never worked; fixed.
* 8sync/agenda.scm (agenda-run-once): Updated for port-remove-request