Add GUC to enable WAL-logging of hint bits, even with checksums disabled.

WAL records of hint bit updates is useful to tools that want to examine
which pages have been modified. In particular, this is required to make
the pg_rewind tool safe (without checksums).

This can also be used to test how much extra WAL-logging would occur if
you enabled checksums, without actually enabling them (which you can't
currently do without re-initdb'ing).

Sawada Masahiko, docs by Samrat Revagade. Reviewed by Dilip Kumar, with
further changes by me.

src/include/access/xlog_internal.h

@@ -209,6 +209,7 @@ typedef struct xl_parameter_change
 	int			max_prepared_xacts;
 	int			max_locks_per_xact;
 	int			wal_level;
+	bool		wal_log_hintbits;
 } xl_parameter_change;
 
 /* logs restore point */