Avoid changing an index's indcheckxmin horizon during REINDEX.

There can never be a need to push the indcheckxmin horizon forward, since
any HOT chains that are actually broken with respect to the index must
pre-date its original creation.  So we can just avoid changing pg_index
altogether during a REINDEX operation.

This offers a cleaner solution than my previous patch for the problem
found a few days ago that we mustn't try to update pg_index while we are
reindexing it.  System catalog indexes will always be created with
indcheckxmin = false during initdb, and with this modified code we should
never try to change their pg_index entries.  This avoids special-casing
system catalogs as the former patch did, and should provide a performance
benefit for many cases where REINDEX formerly caused an index to be
considered unusable for a short time.

Back-patch to 8.3 to cover all versions containing HOT.  Note that this
patch changes the API for index_build(), but I believe it is unlikely that
any add-on code is calling that directly.

src/backend/commands/indexcmds.c

@@ -505,7 +505,7 @@ DefineIndex(RangeVar *heapRelation,
 	indexInfo->ii_BrokenHotChain = false;
 
 	/* Now build the index */
-	index_build(rel, indexRelation, indexInfo, primary);
+	index_build(rel, indexRelation, indexInfo, primary, false);
 
 	/* Close both the relations, but keep the locks */
 	heap_close(rel, NoLock);