ALTER TABLE DISABLE KEYS doesn't work when modifying the table
  
ENABLE|DISABLE KEYS combined with another ALTER TABLE option, different
than RENAME TO did nothing. Also, if the table had disabled keys
and was ALTER-ed then the end table was with enabled keys.
  
Fixed by checking whether the table had disabled keys and enabling them
in the copied table.


myisam/mi_open.c:
  Extend mi_indexes_are_disabled to implement return value
  2 - Non-unique indexes are disabled
mysql-test/r/alter_table.result:
  update result
mysql-test/t/alter_table.test:
  update test
sql/sql_table.cc:
  When ENABLE|DISABLE index is combined with another option
  different than RENAME TO, we should ENABLE|DISABLE the keys of
  the modified table. Also when modifying we should preserve the
  previous state of the indices.
  (This problem exists in 5.0 and 5.1 but since the codebase has
  diverged, this fix won't automerge, but the fix will be quite
  similar).