- When a MyISAM table which belongs to a merge table union and is 
  renamed the associated file descriptors are closed on windows.
  This causes a server crash next time an insert or update is 
  performed on the merge table.
- This patch prevents the system from crashing on windows by
  checking for bad file descriptors every time the MyISAM table
  is locked by associated the merge table.


myisam/mi_locking.c:
  - Added check for bad file descriptors when table is part of merge union.
  - This patch prevents the server from crash on windows.
myisam/myisamdef.h:
  Added boolean value to indicate that this myisam table is part of
  a merge union.
myisammrg/myrg_locking.c:
  Added paramter owned_by_merge=TRUE for mi_lock_database through MYRG_TABLE struct.