Add pg_xlogdump contrib program

This program relies on rm_desc backend routines and the xlogreader
infrastructure to emit human-readable rendering of WAL records.

Author: Andres Freund, with many reworks by Álvaro
Reviewed (in a much earlier version) by Peter Eisentraut

contrib/pg_xlogdump/rmgrdesc.h

@@ -0,0 +1,21 @@
+/*
+ * rmgrdesc.h
+ *
+ * pg_xlogdump resource managers declaration
+ *
+ * contrib/pg_xlogdump/rmgrdesc.h
+ */
+#ifndef RMGRDESC_H
+#define RMGRDESC_H
+
+#include "lib/stringinfo.h"
+
+typedef struct RmgrDescData
+{
+	const char *rm_name;
+	void	  (*rm_desc) (StringInfo buf, uint8 xl_info, char *rec);
+} RmgrDescData;
+
+extern const RmgrDescData RmgrDescTable[];
+
+#endif /* RMGRDESC_H */