agenda: Use suspendable ports feature from Guile 2.2.

agenda: Use suspendable ports feature from Guile 2.2.

This removes the old method of watching for when ports are updated via
select and explicitly calling a particular function, and instead can
simply suspend at the appropriate points using Guile's common I/O
procedures; the agenda handles waking things up when they're ready.

* 8sync/agenda.scm: Import `(ice-9 suspendable-ports)' and run
  install-suspendable-ports!
  (<agenda>, agenda-except-port-map, make-agenda): Drop except-port-map
  from agenda.  The "meaning" of read-port-map and write-port-map has
  changed a bit too; these are still a hash-table of a port to a procedure
  to be called when that port is available, but after being selected,
  items are removed from the table.
  (<port-request>, make-port-request, port-request, <port-remove-request>)
  (port-remove-request, 8sync-port, 8sync-port-remove)
  (agenda-handle-port-request!, agenda-handle-port-remove-request!): Remove.
  (8sync-nowait): Move comment that was previously in 8sync-port here.
  (update-agenda-from-select!): Update to use the new "meaning" of the
  agenda-read-port-map and agenda-write-port-map and to drop referencing
  the deprecated agenda-except-port-map.  Properly removes items from
  the read and write maps once they've been selected.
  (<read-request>, <write-request>, agenda-handle-read-request)
  (agenda-handle-write-request, wait-for-readable, wait-for-writable):
  New variables.
  (stop-on-nothing-to-do): Remove check on removed agenda-except-port-map.
  (agenda-run-once): Parameterize the current-read-waiter and
  current-write-waiter to wait-for-readable and wait-for-writable
  respectively.  Update handle-individual to use the new <read-request>
  and <write-request> types.