The parsing error of the multitoken argument with negative numbers

The parsing error of the multitoken argument with negative numbers occurs while setting negative numbers after the first number.

For example, if we set an option
```cpp
("point,p", po::value>(&options.point)->multitoken(), "The point")
```
and enter in the command line the following values
```
prog -p -1 2 3
```
It works correctly.
However, if we enter the values given bellow
```
prog -p -1 -2 3
```
the parsing error occurs
```
unrecognised option '-2'
```

This error exists in Boost 1.65.1 as well.

使用所示的多 token 选项和两个命令行示例重现该问题，然后跟踪将在第一个 token 之后对负值进行分类的命令行选项解析路径。完成的标准是接受 `-p -1 -2 3` 调用并生成与 `-p -1 2 3` 相同的向量，同时为报告的回归问题提供覆盖。

由索引模型根据 Issue 内容生成。