Thanks for the golang SysProcAttr reference.  The internals of _posixsubprocess have already becoming unwieldy with the abundance of args.  Such a struct/object would also fit in well there and avoid excessive C stack use.  (as izbyshev noted during the vfork work)

Most prctl uses I noticed were PDEATHSIG but I'd need to explicitly audit those.  Users don't seem to care about it's documented main thread caveat (which matches what I've seen; most programs don't use non-daemon threads and exit the main thread).

I want what we do for this to be futureproof for the syscall so that we don't wind up merely picking one feature such as PDEATHSIG to pass a flags through to and needing to add logic to support others later on, delaying the ability to use new system features.