Improve code around the recently added rm_identify rmgr callback.

There are four weaknesses in728f152e07f998d2cb4fe5f24ec8da2c3bda98f2:

* append_init() in heapdesc.c was ugly and required that rm_identify
  return values are only valid till the next call. Instead just add a
  couple more switch() cases for the INIT_PAGE cases. Now the returned
  value will always be valid.
* a couple rm_identify() callbacks missed masking xl_info with
  ~XLR_INFO_MASK.
* pg_xlogdump didn't map a NULL rm_identify to UNKNOWN or a similar
  string.
* append_init() was called when id=NULL - which should never actually
  happen. But it's better to be careful.

src/include/access/xlog_internal.h

@@ -244,9 +244,6 @@ struct XLogRecord;
  * "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