[$] Race-free process creation in the GNU C Library

The pidfd API has been added to the kernel
over the last several years to provide a race-free way for processes to
refer to each other. While the GNU C Library (glibc) gained
basic pidfd support with the 2.36 release in 2022, it still lacks a
complete solution for race-free process creation. This
patch set from Adhemerval Zanella seems likely to fill that gap in the
near future, though, with an extension to the posix_spawn()
API.