low myisam_sort_buffer_size

Repair by sort (default) or parallel repair of a MyISAM table
(doesn't matter partitioned or not) as well as bulk inserts
and enable indexes some times didn't failover to repair with
key cache.

The problem was that after unsuccessful attempt, data file was
closed. Whereas repair with key cache requires open data file.
Fixed by reopening data file.

Also fixed a valgrind warning, which may appear during repair
by sort or parallel repair with certain myisam_sort_buffer_size
number of rows and length of an index entry (very dependent).

mysql-test/r/myisam.result:
  A test case for BUG#47073.
mysql-test/t/myisam.test:
  A test case for BUG#47073.
storage/myisam/ha_myisam.cc:
  Reverted fix for BUG25289. Not needed anymore.
storage/myisam/mi_check.c:
  Reopen data file, when repair by sort or parallel repair
  fails.
  
  When repair by sort is requested to rebuild data file, data file
  gets rebuilt while fixing first index. When rebuild is completed,
  info->dfile is pointing to temporary data file, original data file
  is closed.
  
  It may happen that repair has successfully fixed first index and
  rebuilt data file, but failed to fix second index. E.g.
  myisam_sort_buffer_size was big enough to fix first shorter index,
  but not enough to fix subsequent longer index.
  
  In this case we end up with info->dfile pointing to temporary file,
  which is removed and info->dfile is set to -1.
  
  Though repair by sort failed, the upper layer may still want to
  try repair with key cache. But it needs info->dfile pointing to
  valid data file.
storage/myisam/sort.c:
  When performing a copy of IO_CACHE structure, current_pos and
  current_end must be updated separatly to point to memory we're
  copying to (not to memory we're copying from).
  
  As t_file2 is always WRITE cache, proper members are write_pos
  and write_end accordingly.