bin: Refactoring.

- bin/nbtdebug -> bin/minbtd.
- bin/serverlistdebug -> bin/mipingd.
README: Split Features & Tools.
bin/minbtd: Add cmd docs + ...

- Better logging.
- Reduce var pollution: Flags, flags, flag -> Flags, flag.
bin/mipingd: Add cmd docs + ...

- Use proto/ping instead of byte streams.
bin/miproxy: Little refactor.

- Reduce var pollution: Flags, flags, flag -> Flags, flag.