> Where would be the best place to put these non-POSIX calls?
> 
> I looked at posixmodule.c and it's a mess; much conditional CPP logic
> governing what gets compiled, not clear where I should add something
> like this there - if I should at all, since these routines are not POSIX
> routines.

Don't worry about that - the POSIX module is the right place, despite
it's name.

> Perhaps there should be a module called Unix or something?

That wouldn't reduce the need to remove CPP logic. I personally don't
find that CPP logic very messy - most of it is fairly clear (perhaps
with popen being the exception).

> Also, knowing whether the functions were avaiable at compile time would
> be tricky; some Unix OSes have them and others don't.

I don't understand. When you compile for a specific Unix, it either has
them or not, right? So you *can* test at compile time, and easily so
(the same way it test for about 20 other functions).

> It sounds like a
> job for autoconf to define HAVE_SETRESUID and other CPP definitions like
> that so we can compile cleanly and portably...

Correct - you need to change configure.in as well.