[System] Added missing ctor to Socket for initializing dual-mode socket
 - Ref: https://bugzilla.xamarin.com/show_bug.cgi?id=20048
 - Mono currently doesn't support 2-args Socket ctor added in .NET 4.5
 - Absence of this ctor prevents KestrelHttpServer (web server for
   ASP.NET vNext: https://github.com/aspnet/KestrelHttpServer ) test
   code from working
 - I hesitated to put IPv4 only implementation, but full dual stack
   (IPv4/v6) support would need bunch of works to be done. So, I pinned
   supported protocol of new ctor to IPv4. Also, I filed v4/v6 support
   issue as a MonoTODO entry inside the source code.