Problem: logging queries not using indexes we check a special flag which 
is set only at the server startup and is not changing with a corresponding
server variable together.

Fix: check the variable value instead of the flag.



mysql-test/r/show_check.result:
  Fix for bug #28808: log_queries_not_using_indexes variable dynamic change is ignored
    - test result.
mysql-test/t/show_check.test:
  Fix for bug #28808: log_queries_not_using_indexes variable dynamic change is ignored
    - test case.
sql/mysqld.cc:
  Fix for bug #28808: log_queries_not_using_indexes variable dynamic change is ignored
    - SPECIAL_LOG_QUERIES_NOT_USING_INDEXES is not used anymore.
sql/sql_parse.cc:
  Fix for bug #28808: log_queries_not_using_indexes variable dynamic change is ignored
    - check opt_log_queries_not_using_indexes instead of 
      SPECIAL_LOG_QUERIES_NOT_USING_INDEXES flag.
sql/unireg.h:
  Fix for bug #28808: log_queries_not_using_indexes variable dynamic change is ignored
    - SPECIAL_LOG_QUERIES_NOT_USING_INDEXES is not used anymore.