@sql/mysqld.cc:
- Allow all the modes.

@sql/sql_class.h:
- Update comment.

@sql/sys_vars.cc, @sql/sys_vars.h:
- Combine the check and update functions for the gtid_mode sys_var. This is necessary since locks must be held from check until update without interruption.
- Do not yet implement any checks for gtid_mode: that will be done in subsequent commits.
- Force a binlog rotation when setting gtid_mode.
- Make gtid_mode settable.