described in Mantis#30. Specifically:

- Allow innodb_file_format to take string arguments
- Make innodb_file_format system variable a string instead of a number
- Implement the callback functions
- Update warning messages

Three new functions are implemented:

file_format_name_lookup(): Validate the file format name and return
its corresponding id.

innodb_file_format_check(): Check if it is a valid file format. This
function is registered as a callback with MySQL.

innodb_file_format_update(): Update the global variable using the
"saved" value. This functions is registered as a callback with MySQL.