Patch: add psql tab completion for event triggers

As it was kind of annoying not to have this when playing around with
event triggers.

This also tightens up the existing tab completion for ALTER TRIGGER,
which contained redundant code for table name completion, and which was
also causing a spurious "RENAME TO" to be inserted in this context:

     CREATE EVENT TRIGGER foo ON {event} ^I


Regards

Ian Barwick

--
  Ian Barwick                   http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services