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/backend/parser/parse_param.c

@@ -57,6 +57,7 @@ static Node *variable_coerce_param_hook(ParseState *pstate, Param *param,
 						   Oid targetTypeId, int32 targetTypeMod,
 						   int location);
 static bool check_parameter_resolution_walker(Node *node, ParseState *pstate);
+static bool query_contains_extern_params_walker(Node *node, void *context);
 
 
 /*
@@ -316,3 +317,38 @@ check_parameter_resolution_walker(Node *node, ParseState *pstate)
 	return expression_tree_walker(node, check_parameter_resolution_walker,
 								  (void *) pstate);
 }
+
+/*
+ * Check to see if a fully-parsed query tree contains any PARAM_EXTERN Params.
+ */
+bool
+query_contains_extern_params(Query *query)
+{
+	return query_tree_walker(query,
+							 query_contains_extern_params_walker,
+							 NULL, 0);
+}
+
+static bool
+query_contains_extern_params_walker(Node *node, void *context)
+{
+	if (node == NULL)
+		return false;
+	if (IsA(node, Param))
+	{
+		Param	   *param = (Param *) node;
+
+		if (param->paramkind == PARAM_EXTERN)
+			return true;
+		return false;
+	}
+	if (IsA(node, Query))
+	{
+		/* Recurse into RTE subquery or not-yet-planned sublink subquery */
+		return query_tree_walker((Query *) node,
+								 query_contains_extern_params_walker,
+								 context, 0);
+	}
+	return expression_tree_walker(node, query_contains_extern_params_walker,
+								  context);
+}