Non-conformant nuget package folder layout

[glopesdev]

The recommended folder layout for redistributing native binaries which depend on msbuild props and targets file is to use the build\native folder: https://devblogs.microsoft.com/nuget/native-support/.

The current ZeroMQ nuget package layout places platform-specific folders in the root of the build folder. These should be moved into build\native. Following the standard would help integration with languages and frameworks expecting standard nuget package layouts.