A Query in the MyISAM merge table was crashing 
if the index merge algorithm was being used

Index Merge optimization requires the reading of 
multiple indexes at the same time. Reading multiple 
indexes at once with current SE API means that we 
need to have handler instance for each to-be-read 
index. This is done by creating clones of the handlers 
instances. The clone internally does a open of the handler.

The open for a MERGE engine is handled in the following 
phases

1) open parent table
2) generate list of underlying
   table
3) attach underlying tables

But the current implementation does only the first 
phase (i.e.) open parent table.

The current patch fixes this at the MERGE engine level, 
by handling the clone operation within the MERGE engine 
rather than in the storage engine API. It opens and 
attaches the MyISAM tables on the MyISAM storage engine 
interface directly within the MERGE engine. The new MyISAM 
table instances, as well as the MERGE clone itself, are not 
visible in the table cache. This is not a problem because
all locking is handled by the original MERGE table from which
this is cloned of.

mysql-test/r/merge.result:
  updated the result file to reflect the new tests
  added to test the fix
mysql-test/t/merge.test:
  Added new tests to verify that the index merge
  algorithm does not crash in the merge engine.
storage/myisammrg/ha_myisammrg.cc:
  Implement the clone method, that handles
  
  1) Cloning the handler
  2) Opening underlying MYISAM child tables
  3) Copies the state of the original handler and the children
     into the cloned instances
  4) Sets the appropriate flags
storage/myisammrg/ha_myisammrg.h:
  Added a flag that is set to indicate that the current 
  instance is cloned. Also added the prototype or the clone 
  method.
storage/myisammrg/myrg_open.c:
  Since we do now again use myrg_open() in the server
  removed the comments marking this as deadcode.