Clean up the mess around EXPLAIN and materialized views.

Revert the matview-related changes in explain.c's API, as per recent
complaint from Robert Haas.  The reason for these appears to have been
principally some ill-considered choices around having intorel_startup do
what ought to be parse-time checking, plus a poor arrangement for passing
it the view parsetree it needs to store into pg_rewrite when creating a
materialized view.  Do the latter by having parse analysis stick a copy
into the IntoClause, instead of doing it at runtime.  (On the whole,
I seriously question the choice to represent CREATE MATERIALIZED VIEW as a
variant of SELECT INTO/CREATE TABLE AS, because that means injecting even
more complexity into what was already a horrid legacy kluge.  However,
I didn't go so far as to rethink that choice ... yet.)

I also moved several error checks into matview parse analysis, and
made the check for external Params in a matview more accurate.

In passing, clean things up a bit more around interpretOidsOption(),
and fix things so that we can use that to force no-oids for views,
sequences, etc, thereby eliminating the need to cons up "oids = false"
options when creating them.

catversion bump due to change in IntoClause.  (I wonder though if we
really need readfuncs/outfuncs support for IntoClause anymore.)

src/include/commands/explain.h

@@ -47,7 +47,6 @@ typedef void (*ExplainOneQuery_hook_type) (Query *query,
 													   IntoClause *into,
 													   ExplainState *es,
 													 const char *queryString,
-													   DestReceiver *dest,
 													   ParamListInfo params);
 extern PGDLLIMPORT ExplainOneQuery_hook_type ExplainOneQuery_hook;
 
@@ -68,8 +67,8 @@ extern void ExplainOneUtility(Node *utilityStmt, IntoClause *into,
 				  const char *queryString, ParamListInfo params);
 
 extern void ExplainOnePlan(PlannedStmt *plannedstmt, IntoClause *into,
-			   ExplainState *es, const char *queryString,
-			   DestReceiver *dest, ParamListInfo params);
+			   ExplainState *es,
+			   const char *queryString, ParamListInfo params);
 
 extern void ExplainPrintPlan(ExplainState *es, QueryDesc *queryDesc);