Add new OID alias type regdatabase.

This provides a convenient way to look up a database's OID.  For
example, the query

    SELECT * FROM pg_shdepend
    WHERE dbid = (SELECT oid FROM pg_database
                  WHERE datname = current_database());

can now be simplified to

    SELECT * FROM pg_shdepend
    WHERE dbid = current_database()::regdatabase;

Like the regrole type, regdatabase has cluster-wide scope, so we
disallow regdatabase constants from appearing in stored
expressions.

Bumps catversion.

Author: Ian Lawrence Barwick <barwick@gmail.com>
Reviewed-by: Greg Sabino Mullane <htamfids@gmail.com>
Reviewed-by: Jian He <jian.universality@gmail.com>
Reviewed-by: Fabrízio de Royes Mello <fabriziomello@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/aBpjJhyHpM2LYcG0%40nathan

contrib/postgres_fdw/deparse.c

@@ -39,6 +39,7 @@
 #include "catalog/pg_aggregate.h"
 #include "catalog/pg_authid.h"
 #include "catalog/pg_collation.h"
+#include "catalog/pg_database.h"
 #include "catalog/pg_namespace.h"
 #include "catalog/pg_operator.h"
 #include "catalog/pg_opfamily.h"
@@ -455,6 +456,11 @@ foreign_expr_walker(Node *node,
 											  AuthIdRelationId, fpinfo))
 								return false;
 							break;
+						case REGDATABASEOID:
+							if (!is_shippable(DatumGetObjectId(c->constvalue),
+											  DatabaseRelationId, fpinfo))
+								return false;
+							break;
 					}
 				}