Remove BusyBox flock fallback, require util-linux

BusyBox flock returns 'Bad file descriptor' on NFS mounts and cannot
be used reliably. Instead of silently falling back to a retry loop,
fail fast with a clear error message explaining how to fix it.

- Add _check_flock_support() that detects BusyBox and exits with error
- Check runs at startup for get/put/cleanup commands
- Simplify _flock_with_timeout() to just use -w flag directly
- Update documentation to reflect util-linux requirement