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/backend/access/rmgrdesc/xlogdesc.c

@@ -139,7 +139,7 @@ xlog_identify(uint8 info)
 {
 	const char *id = NULL;
 
-	switch (info)
+	switch (info & ~XLR_INFO_MASK)
 	{
 		case XLOG_CHECKPOINT_SHUTDOWN:
 			id = "CHECKPOINT_SHUTDOWN";