Unix: fix UDP ReuseAddress with non .NET Core UDP clients (dotnet/corefx#32046)

Unix: fix UDP ReuseAddress with non .NET Core UDP clients (dotnet/corefx#32046)

* Unix: fix UDP ReuseAddress with non .NET Core UDP clients

.NET Core sets SO_REUSEPORT to enable reusing the address with other
sockets. This works when the other sockets also set SO_REUSEPORT.

SO_REUSEADDR can also be used for this. When one application uses
SO_REUSEADDR and another uses SO_REUSEPORT, the second application
will fail to bind.

To implement SocketOptionName.ReuseAddress we need to set both options.
This enables sharing with applications that set SO_REUSEPORT,
SO_REUSEADDR or both.

* Fix comment

* wording: share -> reuse

* Add test

* Fix test on mac

* Move DllImport to separate file

* Add separate csproj Configurations for Windows and Unix

* Add Windows and Unix to Configurations.props

Commit migrated from https://github.com/dotnet/corefx/commit/0ec9c9d7caac1ac378e2135620bf2f8b80e6ed86