Rename user-facing tools with "xlog" in the name to say "wal".

This means pg_receivexlog because pg_receivewal, pg_resetxlog
becomes pg_resetwal, and pg_xlogdump becomes pg_waldump.

src/backend/access/rmgrdesc/xactdesc.c

@@ -26,7 +26,7 @@
  * understand format.
  *
  * This routines are in xactdesc.c because they're accessed in backend (when
- * replaying WAL) and frontend (pg_xlogdump) code. This file is the only xact
+ * replaying WAL) and frontend (pg_waldump) code. This file is the only xact
  * specific one shared between both. They're complicated enough that
  * duplication would be bothersome.
  */

src/backend/access/transam/varsup.c

@@ -399,7 +399,7 @@ SetTransactionIdLimit(TransactionId oldest_datfrozenxid, Oid oldest_datoid)
  *
  * We primarily check whether oldestXidDB is valid.  The cases we have in
  * mind are that that database was dropped, or the field was reset to zero
- * by pg_resetxlog.  In either case we should force recalculation of the
+ * by pg_resetwal.  In either case we should force recalculation of the
  * wrap limit.  Also do it if oldestXid is old enough to be forcing
  * autovacuums or other actions; this ensures we update our state as soon
  * as possible once extra overhead is being incurred.

src/backend/access/transam/xlogutils.c

@@ -647,7 +647,7 @@ XLogTruncateRelation(RelFileNode rnode, ForkNumber forkNum,
  * always be one descriptor left open until the process ends, but never
  * more than one.
  *
- * XXX This is very similar to pg_xlogdump's XLogDumpXLogRead and to XLogRead
+ * XXX This is very similar to pg_waldump's XLogDumpXLogRead and to XLogRead
  * in walsender.c but for small differences (such as lack of elog() in
  * frontend).  Probably these should be merged at some point.
  */