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/nodes/primnodes.h

@@ -82,6 +82,10 @@ typedef struct RangeVar
 /*
  * IntoClause - target information for SELECT INTO, CREATE TABLE AS, and
  * CREATE MATERIALIZED VIEW
+ *
+ * For CREATE MATERIALIZED VIEW, viewQuery is the parsed-but-not-rewritten
+ * SELECT Query for the view; otherwise it's NULL.  (Although it's actually
+ * Query*, we declare it as Node* to avoid a forward reference.)
  */
 typedef struct IntoClause
 {
@@ -92,8 +96,8 @@ typedef struct IntoClause
 	List	   *options;		/* options from WITH clause */
 	OnCommitAction onCommit;	/* what do we do at COMMIT? */
 	char	   *tableSpaceName; /* table space to use, or NULL */
+	Node	   *viewQuery;		/* materialized view's SELECT query */
 	bool		skipData;		/* true for WITH NO DATA */
-	char		relkind;		/* RELKIND_RELATION or RELKIND_MATVIEW */
 } IntoClause;