Split out list of XLog resource managers

The new rmgrlist.h header, containing all necessary data
about built-in resource managers, allows other pieces of code to
access them.

In particular, this allows a future pg_xlogdump program to extract
rm_desc function pointers, without having to keep a duplicate list of
them.

src/include/access/xlog_internal.h

@@ -233,7 +233,10 @@ struct XLogRecord;
 /*
  * Method table for resource managers.
  *
- * RmgrTable[] is indexed by RmgrId values (see rmgr.h).
+ * This struct must be kept in sync with the PG_RMGR definition in
+ * rmgr.c.
+ *
+ * RmgrTable[] is indexed by RmgrId values (see rmgrlist.h).
  */
 typedef struct RmgrData
 {