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At this chance, the API is simplified in the following ways:

- we don't provide pulse-mode semaphores anymore. Those should be
  provided by a higher level API as flags for instance (ungated
  monitors would support that easily by using the signedness bit of an
  event monitor value as such flag).

- threads now always wait by priority order. The rarely used FIFO wait
  mode was dropped.

Those changes make the @flags parameter to evl_new_sem() useless,
which allows to remove it from the signature.