Having a hardcoded relative path worked okay with the nmake Makefile
build but cause problems on a visual studio build. The CI caught that
right away.
Remove the relative paths, and add additional include directories for
the resouce compiler to search in the Makefile and in visual studio.
rename sys/winnt to sys/windows
move vs (visual studio) folder out of win/win32 and into sys/windows
rename include/ntconf.h to include/windconf.h
rename winnt.c to windsys.c
place visual studio projects into individual subfolders.
This will hopefully resolve GitHub issue #484 as well.
