gspawn: Provide less brute force fdwalk implementation on macOS
It seems macOS has a way to query open fds for a process, so this commit tries to use it, instead of just trying all fds between 0 and $BIGNUMBER.
It seems macOS has a way to query open fds for a process, so this commit tries to use it, instead of just trying all fds between 0 and $BIGNUMBER.