Add support for setting a --maximum value for session variables

[michaelcoburn]

The --maximum prefix is available for mysqld only and permits a limit to be placed on how large client programs can set session system variables. To do this, use a --maximum prefix with the variable name. For example, --maximum-max_heap_table_size=32M prevents any client from making the heap table size limit larger than 32M.

The --maximum prefix is intended for use with system variables that have a session value. If applied to a system variable that has only a global value, an error occurs. For example, with --maximum-back_log=200, the server produces this error:

Maximum value of 'back_log' cannot be set