Fixed rare bug in MYISAM introduced in 4.0.3 where the index file header was not updated directly after an UPDATE of split dynamic rows.


myisam/mi_locking.c:
  Added DBUG info
myisam/mi_open.c:
  Added DBUG info
myisam/mi_update.c:
  More comments
  Fixed rare bug in MYISAM introduced in 4.0.3 where the index file header
  was not updated directly after an UPDATE of split dynamic rows.
mysql-test/r/myisam.result:
  Added test case for MyISAM UPDATE bug
mysql-test/t/myisam.test:
  Added test case for MyISAM UPDATE bug