Update posix_spawn for POSIX 2024

[kib]

Summary

posix_spawn_file_action_addopen.3: ignoring close failure is now approved


posix_spawn: actions chdir and fchdir are now required by POSIX

Drop the _np suffix.

[dim]

Apart from the manpage nits, LGTM.

lib/libc/gen/posix_spawn.3

449

This also needs a posix_spawn_file_actions_addchdir entry.

451

Hm, was this one also standardized?

[kib]

Fix manpage.
Switch test to use non-_np names.

[dim]

LGTM

[ngie]

Thank you @kib !