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

src/include/utils/palloc.h

@@ -28,8 +28,6 @@
 #ifndef PALLOC_H
 #define PALLOC_H
 
-#ifndef FRONTEND
-
 /*
  * Type MemoryContextData is declared in nodes/memnodes.h.	Most users
  * of memory allocation should just treat it as an abstract type, so we
@@ -37,6 +35,8 @@
  */
 typedef struct MemoryContextData *MemoryContext;
 
+#ifndef FRONTEND
+
 /*
  * CurrentMemoryContext is the default allocation context for palloc().
  * We declare it here so that palloc() can be a macro.	Avoid accessing it