No way to get number of non-unlimited positional arguments

[Documentation](https://www.boost.org/doc/libs/1_68_0/doc/html/boost/program_options/positiona_1_3_31_9_9_1_1_1.html) for `positional_options_description` says it has a `max_total_count` function that returns the number of positional arguments, and may return `numeric_limits::max` to indicate unlimited arguments. However, there is no way to determine the number of counted arguments.

Given an instance of `positional_options_description`, it is therefore impossible, for example, to print out a list of all positional arguments, stopping at the last, unlimited argument. A new member function that provides this is all you'd need to add, IMO.

先阅读链接的 positional_options_description 文档，然后在 program_options 模块中找到类的实现及其现有测试。检查 max_total_count 如何区分无限参数。完成标准是公开已计数的位置参数数量，并通过测试和文档覆盖该行为。

由索引模型根据 Issue 内容生成。