agenda: When re-invoking a continuation at the agenda, don't wrap in a catch.

* 8sync/agenda.scm (<kontinue>): Add new record type which just wraps a
delimited continuation.
(8sync, 8sleep, 8yield): Wrap continuations in kontinue.
(agenda-run-once!): If "kontinue?", just resume the continuation without
catching errors (it already has an error handler!).