include: Take care to not declare CtxtHandle and PCtxtHandle more than once when including both sspi.h and wincred.h.

inetcomm: Call InternetTransport_RegisterClass and UnregisterClass on process attach and detach respectively, to register a window class and initialize Winsock.

inetcomm: Use InternetTransport_ChangeStatus in InternetTransport_DropConnection so that the callback gets called.

Based on a patch by Rob Shearman.

inetcomm: Add a framework for connecting to a server and sending/receiving data asynchronously with callbacks being called in the context of the original thread, using window messages.

Reported by Gerald Pfeifer.