Changed the semantics of open_count so that it is decremented
at every unlock (if it was incremented due to data changes).
So it indicates a crash, if it is non-zero after a lock.
The table will then be repaired.


myisam/mi_close.c:
  bug#2831 - --extenral-locking does not fully work with --myisam-recover.
  To avoid flushing the open_count at every unlock,
  we need to do so at close at least.
myisam/mi_locking.c:
  bug#2831 - --extenral-locking does not fully work with --myisam-recover.
  open_count is now decremented at unlock (from a writelock) with
  mi_unlock_open_count(). After every new lock the state is read
  from the index file and the open_count checked. If not zero,
  another server must have crashed, so the table is marked as crashed.
  In certain situations the decremented open_count mut be flushed to
  the index file. I tried to keep these extra flushes as seldom as possible.
sql/ha_myisam.cc:
  bug#2831 - --extenral-locking does not fully work with --myisam-recover.
  Added code to repair the table, if it is marked crashed after
  successful locking and the --myisam-recover option is set.
sql/sql_table.cc:
  This does not really belong to the bugfix for #2831.
  But it was detected during fixing the external locking.