This is a back port from 5.1 to 5.0.

Fix for BUG 20023: mysql_change_user() resets the value
of SQL_BIG_SELECTS.
  
The bug was that SQL_BIG_SELECTS was not properly set
in COM_CHANGE_USER.
  
The fix is to update SQL_BIG_SELECTS properly.



sql/sql_class.cc:
  Update THD::options with the respect to SQL_BIG_SELECTS
  in COM_CHANGE_USER.
tests/mysql_client_test.c:
  Add a test case BUG#20023.