MERGE engine".
Backport the patch from 6.0 by Ingo Struewing:
revid:ingo.struewing@sun.com-20091028183659-6kmv1k3gdq6cpg4d
Bug#36171 - CREATE TEMPORARY TABLE and MERGE engine

In former MySQL versions, up to 5.1.23/6.0.4 it was possible to create
temporary MERGE tables with non-temporary MyISAM tables.
        
This has been changed in the mentioned version due to Bug 19627
(temporary merge table locking). MERGE children were locked through
the parent table. If the parent was temporary, it was not locked and
so the children were not locked either. Parallel use of the MyISAM
tables corrupted them.
       
Since 6.0.6 (WL 4144 - Lock MERGE engine children), the children are
locked independently from the parent. Now it is possible to allow
non-temporary children with a temporary parent. Even though the
temporary MERGE table itself is not locked, each non-temporary
MyISAM table is locked anyway.
        
NOTE: Behavior change: In 5.1.23/6.0.4 we prohibited non-temporary
children with a temporary MERGE table. Now we re-allow it.
An important side-effect is that temporary tables, which overlay
non-temporary MERGE children, overlay the children in the MERGE table.


mysql-test/r/merge.result:
  Update results (Bug#36171).
mysql-test/r/merge_mmap.result:
  Update results (Bug#36171).
mysql-test/t/merge.test:
  Add tests for Bug#36171
mysql-test/t/merge_mmap.test:
  Add tests for Bug#36171.
storage/myisammrg/ha_myisammrg.cc:
  Changed constraint for temporary state of tables.