[RFC] posix: Make system, popen, and wordexp try pidfd_spawn first (BZ 34001)

[Florian Weimer]

* Adhemerval Zanella:

> This patch updates system, popen, and wordexp to use a new internal
> process management abstraction (__spawn_process_create, __spawn_process_wait,
> and __spawn_process_kill) that leverages pidfd_spawn on supported Linux
> kernels before falling back to traditional posix_spawn.

How does this change the way SIGCHLD is delivered?

Thanks,
Florian