event: add new kind of event source called "post"

This new event source is triggered by the dispatching of any non-post
event source. It can thus be used to do clean-up or recheck work,
triggered by any other event source having been executed.

This is different from "defer" event source which are unconditionally
triggered as long as they are enabled. A "defer" event source that does
nothing will result in the event loop busy looping unless it is turned
off eventually. This is different for "post" event sources that will be
only triggered when some other non-post ran, and will thus not keep the
event loop busy on its own.

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