Add code to prevent transaction ID wraparound by enforcing a safe limit

in GetNewTransactionId().  Since the limit value has to be computed
before we run any real transactions, this requires adding code to database
startup to scan pg_database and determine the oldest datfrozenxid.
This can conveniently be combined with the first stage of an attack on
the problem that the 'flat file' copies of pg_shadow and pg_group are
not properly updated during WAL recovery.  The code I've added to
startup resides in a new file src/backend/utils/init/flatfiles.c, and
it is responsible for rewriting the flat files as well as initializing
the XID wraparound limit value.  This will eventually allow us to get
rid of GetRawDatabaseInfo too, but we'll need an initdb so we can add
a trigger to pg_database.

src/include/utils/flatfiles.h

@@ -0,0 +1,33 @@
+/*-------------------------------------------------------------------------
+ *
+ * flatfiles.h
+ *	  Routines for maintaining "flat file" images of the shared catalogs.
+ *
+ *
+ * $PostgreSQL: pgsql/src/include/utils/flatfiles.h,v 1.1 2005/02/20 02:22:07 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef FLATFILES_H
+#define FLATFILES_H
+
+#include "fmgr.h"
+
+extern void database_file_update_needed(void);
+extern void group_file_update_needed(void);
+extern void user_file_update_needed(void);
+
+extern char *database_getflatfilename(void);
+extern char *group_getflatfilename(void);
+extern char *user_getflatfilename(void);
+
+extern void BuildFlatFiles(bool database_only);
+
+extern void AtEOXact_UpdateFlatFiles(bool isCommit);
+extern void AtEOSubXact_UpdateFlatFiles(bool isCommit,
+										SubTransactionId mySubid,
+										SubTransactionId parentSubid);
+
+extern Datum flatfile_update_trigger(PG_FUNCTION_ARGS);
+
+#endif   /* FLATFILES_H */