Add rmgr callback to name xlog record types for display purposes.

This is primarily useful for the upcoming pg_xlogdump --stats feature,
but also allows to remove some duplicated code in the rmgr_desc
routines.

Due to the separation and harmonization, the output of dipsplayed
records changes somewhat. But since this isn't enduser oriented
content that's ok.

It's potentially desirable to further change pg_xlogdump's display of
records. It previously wasn't possible to show the record type
separately from the description forcing it to be in the last
column. But that's better done in a separate commit.

Author: Abhijit Menon-Sen, slightly editorialized by me
Reviewed-By: Álvaro Herrera, Andres Freund, and Heikki Linnakangas
Discussion: 20140604104716.GA3989@toroid.org

src/include/access/xlog_internal.h

@@ -239,6 +239,14 @@ struct XLogRecord;
  * This struct must be kept in sync with the PG_RMGR definition in
  * rmgr.c.
  *
+ * rm_identify must return a name for the record based on xl_info (without
+ * reference to the rmid). For example, XLOG_BTREE_VACUUM would be named
+ * "VACUUM". rm_desc can then be called to obtain additional detail for the
+ * record, if available (e.g. the last block).
+ *
+ * The return value from rm_identify is a pointer to a statically allocated
+ * buffer, and only valid until the next invocation of the callback.
+ *
  * RmgrTable[] is indexed by RmgrId values (see rmgrlist.h).
  */
 typedef struct RmgrData
@@ -246,6 +254,7 @@ typedef struct RmgrData
 	const char *rm_name;
 	void		(*rm_redo) (XLogRecPtr lsn, struct XLogRecord *rptr);
 	void		(*rm_desc) (StringInfo buf, struct XLogRecord *rptr);
+	const char *(*rm_identify) (uint8 info);
 	void		(*rm_startup) (void);
 	void		(*rm_cleanup) (void);
 } RmgrData;