Split out rmgr rm_desc functions into their own files

This is necessary (but not sufficient) to have them compilable outside
of a backend environment.

src/backend/access/rmgrdesc/clogdesc.c

@@ -0,0 +1,41 @@
+/*-------------------------------------------------------------------------
+ *
+ * clogdesc.c
+ *    rmgr descriptor routines for access/transam/clog.c
+ *
+ * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *    src/backend/access/rmgrdesc/clogdesc.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/clog.h"
+
+
+void
+clog_desc(StringInfo buf, uint8 xl_info, char *rec)
+{
+	uint8		info = xl_info & ~XLR_INFO_MASK;
+
+	if (info == CLOG_ZEROPAGE)
+	{
+		int			pageno;
+
+		memcpy(&pageno, rec, sizeof(int));
+		appendStringInfo(buf, "zeropage: %d", pageno);
+	}
+	else if (info == CLOG_TRUNCATE)
+	{
+		int			pageno;
+
+		memcpy(&pageno, rec, sizeof(int));
+		appendStringInfo(buf, "truncate before: %d", pageno);
+	}
+	else
+		appendStringInfo(buf, "UNKNOWN");
+}