closing stdin in a subshell leaks into the parent when posix_spawn() is enabled

[krader1961]

I recently made it possible to build again with ksh using posix_spawn() (see commit be962c7). That results in the basic.sh unit test to fail. I've boiled it down to a minimal test case and can reproduce on macOS 10.14 and Fedora 28. Replace the content of src/cmd/ksh93/tests/basic.sh with this:

set -x
x=$($SHELL -c 'echo yes' <&-)

lsof -p $$
print xxx > $TEST_DIR/file
for x in 1 2
do
    tee >(sleep 1; cat > $TEST_DIR/file) <<< "hello" > /dev/null
    lsof -p $$
done

Then build and test with meson -Duse-spawn=true; ninja; meson test basic. If you inspect build/meson-logs/testlog.txt you should see that the subshell dies from a SIGSEGV when executing the builtin cat. Why this requires a for loop is unknown but removing that loop does not result in failure. The lsof -p $$ aren't required to reproduce the problem. They're only to show that the shells stdin is closed.

Also, the SIGSEGV is because after executing fp = sfstdin; the subsequent use of fp is fatal because sfstdin is NULL due to the problem described above.

[krader1961]

And, of course, this is another example for why I wrote in issue #1117 that stdin, stdout, and stderr should never be left closed. The problem here appears to involve the non-forking optimization described in issue #480 but I don't really know at this stage.