Initial support for MSTcpIp headers and functions

This initial commit provides a minimal header file for some constants and macros provided by Microsoft's MsTcpIp.h file.  More work is necessary to provide definitions of some functions available post-Vista, and probably an import library will need to be created.  However, this commit provides some necessary definitions.

The contents are borrowed from MinGW-w64's public domain implementation.  Any further work will most likely be OpenWatcom-specific.