Implementation of P_DETACH for spawn functions on Windows NT

This pull request implements the P_DETACH constant and functionality for the spawn... functions on Windows NT systems only.  The work is meant to close Issue #20.  The pull request includes documentation updates.

The pull request brings along a minor fix to the QA makefiles for the CLIB so that they pull in the local build headers (please let me know if this is incorrect).

The pull request includes a fix to the declaration of the __p_overlay constant such that it can now be used in dynamic linking situations.