Added a try to a normal repair() if repair_by_sort() failed.
This was not done with ENABLE KEYS and OPTIMIZE TABLE.
Fixed error code handling in mysql_alter_table().


sql/ha_myisam.cc:
  BUG#5625 - MyISAM Index corruption on ALTER TABLE x ENABLE KEYS due to full tmpdir.
  Added a try to a normal repair() if repair_by_sort() failed.
  This was not done with ENABLE KEYS and OPTIMIZE TABLE.
sql/sql_table.cc:
  BUG#5625 - MyISAM Index corruption on ALTER TABLE x ENABLE KEYS due to full tmpdir.
  Added a translation from 'bool' return value to '< 0' error indication,
  which is used within mysql_execute_command() and must as such be
  handed over by mysql_alter_table(). A returncode >= 0 is interpreted
  as 'I have already called send_ok()'.