I planned to put just the file system routines into these files, but
there are starting to be so many of them that I don't want to
duplicate the same set for networking, subprocesses etc.
This implies that OS names have changed. Unix-like OS names are now
capitalized. "macos" is now "Darwin". Windows is now all-lowecase "windows".
"win32" is gone. "win64" was not used in the original code despite a reference
to it.