[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.
Merge pull request #1364 from muojp/addDotNet45_SocketCtor

[System] Added missing ctor to Socket for initializing dual-mode socket
Fix Windows build by including wstcpip.h before winsock.h is included

Error generated by socket-io.c was:

/usr/i686-pc-mingw32/sys-root/mingw/include/ws2tcpip.h:38:2:
error: #error "ws2tcpip.h is not compatible with winsock.h.
Include winsock2.h instead."

Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
[build] Enable dolt on mingw32
Merge pull request #1367 from kornelpal/dolt-mingw32

[build] Enable dolt on mingw32
Merge pull request #1366 from DynamicDevices/ajl/win32build

Fix Windows build by including wstcpip.h before winsock.h is included