The os-xxx.h files are created totally by hand.  Start with os-gen.h,
the generic version of the os.h file.

The os-xxx.c files may be created by hand or you may use the simple include
method that is used here.  The os.c files are made by creating an
os-xxx.ic file which lists all of the files it includes, and then
running make os-xxx.c.  There is an os-gen.ic generic version of the
.ic file.

You will also need to create a makefile.xxx in the parent directory.  There
are other makefiles to inspect there, including a generic makefile.gen.

The names of all the included files here have to be eight characters or less
for DOS, hence the stupid names.