More improvements and fixes to generic config infra

- Adding is_valid_fn and update_fn, both return 1 for success and 0 for failure with an optional error message.
- Bugfix in handling boundary check of unsigned numeric types (was boundaries as signed)
- Adding more numeric types to generic mechanism: uint, ulonglong, long, time_t, off_t
- More verbose error replies ("argument must be between" in out of range CONFIG SET (like config file parsing)
