[Build-Win32] dllmap is not supported by the native .NET impl

The native .NET framework will throw a System.Configuration.ConfigurationErrorsException, dllmaps are a mono extension and absolutely not portable.
For coding, the msbuild-preprocess.ps1 now copies recommended configs for Win32 located inside 'lib/win32'.
After that smuxi-frontend-gnome.exe and smuxi-server.exe work out of the box.