Basic multiplatform

[digivod]

I have static DLLImport projects for iOS and MacCatalyst. They rely on the data structures provided in FFmpeg.AutoGen. Unfortunately static ffmpeg ctor currently fails on those platforms with UnsupportedOperationException. Her's a small fix to prevent that. As a workaround until a final solution is in place.

Also .NET6 as a separate target to allow specific code for the most relevant target framework. This way all platforms can be properly detected. EAGAIN is 35 on BSD style platforms (notably Apple) and 11 on all others.