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.

Perhaps there should be a module called Unix or something?

Also, knowing whether the functions were avaiable at compile time would
be tricky; some Unix OSes have them and others don't.  It sounds like a
job for autoconf to define HAVE_SETRESUID and other CPP definitions like
that so we can compile cleanly and portably...

Thoughts?