for bug#16425: Events: no DEFINER clause.  The problem was that there
were two rules

  ALTER view_algorithm_opt definer ... VIEW ...
  ALTER definer EVENT ...

so when there was 'ALTER definer' in the input it was unclear if empty
view_algorithm_opt should be executed or not.

We solve this by introducing three distinct rules

  ALTER view_algorithm definer ... VIEW ...
  ALTER definer ... VIEW ...
  ALTER definer EVENT ...

that remove the ambiguity.