Pull in the required MonoPoxisHelper method calls into this repo.

Currently for .net 6 and other platforms we have a runtime dependency on Mono.Posix.NetStandard. This is only because we need libMonoPosixHelper.so/dylib on the various platforms. We already pulled in the required C# code so we are independent of that library. So we should also pull in the required native bits.

Now we can either

1. Create a new native lib `libZipSharpHelper` which contains those calls or..
2. Fold that code into the `libzip` native libraries.

Not sure which option is better..

未指定文件或测试。首先跟踪现有 C# 对 Mono.Posix.NetStandard 的使用以及所需的 libMonoPosixHelper.so/dylib 调用，然后比较提议的两个原生库位置。当所需的原生部分能够在受支持的平台上运行且不依赖该运行时依赖项时，即视为完成。

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