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To cope with really slow VM.

This is actually how it behaves, 'event' may be confusing due to
other implementations using different semantics.

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.

evl_log() and evl_printf() helpers moved to a utility file.