Both client and server C code have a lot of historical baggage for
very old UNIX systems.  I'd like to remove some of that, as the
baggage makes ongoing maintenance a little harder than it could be.

For instance, I recently removed RETSIGTYPE from COW because
autoconf's autoupdate suggested it was no longer required.

Pull requests welcome.

I don't wish to appear to be focused on Linux, but I only test on
Linux so if you have a favourite UNIX or UNIX-like operating system,
please get involved.