Pull in the required MonoPoxisHelper method calls into this repo.

[dellis1972]

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..

[grendello]

Mono.Posix + the helper native lib will soon be moving to their own repository. Perhaps we could submodule it and refactor the native helper code so that we can include the sources we need in our build?