optimization: now can use index to find records to update/delete
when there is no WHERE clause.


mysql-test/r/update.result:
  Testcase for BUG#12915
mysql-test/t/update.test:
  Testcase for BUG#12915
sql/mysql_priv.h:
  BUG#12915: Added init_read_record_idx function.
sql/opt_range.cc:
  BUG#12915: Added get_index_for_order() - find an index that can be
  used to get first N table records in given ordering cheaper then 
  one would with full table scan.
sql/opt_range.h:
  BUG#12915: Added get_index_for_order() function
sql/records.cc:
  BUG#12915: Added init_read_record_idx(), rr_index() that allow to scan
  index using init_read_record()/read_record.read_record()
sql/sql_delete.cc:
  BUG#12915: Added single-table DELETE ... ORDER BY ... LIMIT optimization: 
  now can use index to find records to delete when there is no WHERE clause.
sql/sql_update.cc:
  BUG#12915: Added single-table UPDATE ... ORDER BY ... LIMIT optimization: 
  now can use index to find records to update when there is no WHERE clause.
sql/structs.h:
  BUG#12915: Added init_read_record_idx(), rr_index() that allow to scan
  index using init_read_record()/READ_RECORD::read_record()