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/mysql_priv.h:
  Cleanup: make prepare_new_connection_state() private for module.
sql/sql_class.cc:
  Update THD::options with the respect to SQL_BIG_SELECTS
  in COM_CHANGE_USER.
sql/sql_connect.cc:
  Cleanup: make prepare_new_connection_state() private for module.
tests/mysql_client_test.c:
  Add a test case BUG#20023.