On Windows we use WSASend() to send iovecs.

There is a lot of type mangling going on there for the error check and eventual return type of mongoc_socket_try_sendv().

This needs to be updated to use clean types and explicit error checking.

We also need to check the result for SOCKET_ERROR as per the docs: https://msdn.microsoft.com/en-us/library/windows/desktop/ms742203%28v=vs.85%29.aspx