Step 1 in modernization of the Win32 module. Eliminate Windows NT based Windows checks, since that is all we support now. Eliminate thread limits by eliminating thread arrays and switching to using Thread Local Storage (TLS). Simplify dw_clipboard_get_text() by returning a malloc()ed buffer which now must need to be dw_free()ed. This makes this function the same as it does on OS/2 and Mac currently.