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/smgrdesc.c

@@ -0,0 +1,45 @@
+/*-------------------------------------------------------------------------
+ *
+ * smgrdesc.c
+ *    rmgr descriptor routines for catalog/storage.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/smgrdesc.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "catalog/catalog.h"
+#include "catalog/storage_xlog.h"
+
+
+void
+smgr_desc(StringInfo buf, uint8 xl_info, char *rec)
+{
+	uint8		info = xl_info & ~XLR_INFO_MASK;
+
+	if (info == XLOG_SMGR_CREATE)
+	{
+		xl_smgr_create *xlrec = (xl_smgr_create *) rec;
+		char	   *path = relpathperm(xlrec->rnode, xlrec->forkNum);
+
+		appendStringInfo(buf, "file create: %s", path);
+		pfree(path);
+	}
+	else if (info == XLOG_SMGR_TRUNCATE)
+	{
+		xl_smgr_truncate *xlrec = (xl_smgr_truncate *) rec;
+		char	   *path = relpathperm(xlrec->rnode, MAIN_FORKNUM);
+
+		appendStringInfo(buf, "file truncate: %s to %u blocks", path,
+						 xlrec->blkno);
+		pfree(path);
+	}
+	else
+		appendStringInfo(buf, "UNKNOWN");
+}