Allow the audit log to be rotated when MySQL starts

Whilst there are workarounds, e.g. used a format-specific extension for audit_log_file and make sure it matches if you change audit_log_format, it would be nice if you could make sure that the log was rotated when the server starts, e.g. audit_log_rotate_onboot

.. you end up with mangled logs if you change format and forget to rotate manually, etc