Cover pdnew, pdsetfd, and pdstart: basic spawn and wait
(exit status of both true and false), installing and replacing
stdin/stdout descriptors with pdsetfd, interpreter scripts,
destroying an embryonic process by closing its descriptor before
pdstart, and the ENOEXEC/EBADF error cases.
Note: This patch series overall is very big, more than I feel competent
reviewing, so I feel a bit ill-mannered submitting it yet. Certainly,
everyone should feel free to ignore it until my "unix socket connectat"
series is landed. I am submitting it now anyway simply because I think
the end functionality is very cool, and I don't want it to grow dusty on
my machine.
Assisted-by: Claude Code (Claude Opus 4.8/5 and Fable 5)