doesn't recognize it

This is a 5.0 version of the patch, it will be null-merged to 5.1

Problem:

'log' and 'log_slow_queries' were "fixed" variables, i.e. they showed up
in SHOW VARIABLES, but could not be used in expressions like 
"select @@log". Also, using them in the SET statement produced an 
incorrect "unknown system variable" error.

Solution:

Make 'log' and 'log_slow_queries' read-only dynamic variables to make 
them available for use in expressions, and produce a correct error 
about the variable being read-only when used in the SET statement.


mysql-test/r/variables.result:
  Added a test case for bug #29131.
mysql-test/t/variables.test:
  Added a test case for bug #29131.
sql/mysql_priv.h:
  Changed the type of opt_log and opt_slow_log to my_bool to 
  align with the interfaces in set_var.cc
sql/mysqld.cc:
  Changed the type of opt_log and opt_slow_log to my_bool to 
  align with the interfaces in set_var.cc
sql/set_var.cc:
  Made 'log' and 'log_slow_queries' to be read-only dynamic system 
  variable, i.e. available for use in expressions with the @@var syntax.
sql/set_var.h:
  Added a new system variable class representing a read-only boolean
  variable.