native: Ignore stdio fds with /dev/null

When spawning a process, stdio file descriptors can be configured to be ignored,
which basically means that they'll be closed. Currently this is done by
literally closing the file descriptors in the child, but this can have adverse
side effects if the child process then opens a new file descriptor, assigning it
to a stdio number.

To work around the problems of the child, this commit alters the process
spawning code to map stdio fds to /dev/null on unix when they are specified as
being ignored. This should allow spawned programs to have more expected behavior
when opening new files.

Closes # -- 14456