Make new event trigger facility actually do something.

Commit 3855968f32 added syntax, pg_dump,
psql support, and documentation, but the triggers didn't actually fire.
With this commit, they now do.  This is still a pretty basic facility
overall because event triggers do not get a whole lot of information
about what the user is trying to do unless you write them in C; and
there's still no option to fire them anywhere except at the very
beginning of the execution sequence, but it's better than nothing,
and a good building block for future work.

Along the way, add a regression test for ALTER LARGE OBJECT, since
testing of event triggers reveals that we haven't got one.

Dimitri Fontaine and Robert Haas

src/backend/executor/spi.c

@@ -1912,9 +1912,9 @@ _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
 				ProcessUtility(stmt,
 							   plansource->query_string,
 							   paramLI,
-							   false,	/* not top level */
 							   dest,
-							   completionTag);
+							   completionTag,
+							   PROCESS_UTILITY_QUERY);
 
 				/* Update "processed" if stmt returned tuples */
 				if (_SPI_current->tuptable)