[PATCH][BZ 21340] add support for POSIX_SPAWN_SETSID

[Daurnimator]

(replying to self as I'm not subbed to list, and only by chance saw
Adhemerval Zanella's reply via patchwork)

> It lacks the proper Changelog, but patch looks good.  Could you provide
> one? Also, I assume you checked at least in one architecture (we usually
> indicate which system we ran the patch and the results).

Will send over new patch.
Tested on linux on x64.
I've also now written a test case for POSIX_SPAWN_SETSID.