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/commands/schemacmds.c

@@ -132,9 +132,9 @@ CreateSchemaCommand(CreateSchemaStmt *stmt, const char *queryString)
 		ProcessUtility(stmt,
 					   queryString,
 					   NULL,
-					   false,	/* not top level */
 					   None_Receiver,
-					   NULL);
+					   NULL,
+					   PROCESS_UTILITY_SUBCOMMAND);
 		/* make sure later steps can see the object created here */
 		CommandCounterIncrement();
 	}