hush: fix a bug in FEATURE_SH_STANDALONE=y config. Closes 9186

Run this in a "sh SCRIPT":

sha256sum /dev/null
echo END

sha256sum is a NOEXEC applet. It runs in a forked child. Then child exit()s.
By this time, entire script is read, and buffered in a FILE object
from fopen("SCRIPT"). But fgetc() did not consume entire input.
exit() lseeks back by -9 bytes, from <eof> to 'e' in 'echo'.
(this may be libc-specific).
This change of fd position *is shared with the parent*!

Now parent can read more, and it thinks there is another "echo END".
End result: two "echo END"s are run.

Fix this by _exit()ing instead.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush_test/run-all

@@ -64,11 +64,12 @@ do_test()
 	echo -n "$1/$x:"
 	(
 		"$THIS_SH" "./$x" >"$name.xx" 2>&1
+		r=$?
 		# filter C library differences
 		sed -i \
 			-e "/: invalid option /s:'::g" \
 			"$name.xx"
-		test $? -eq 77 && rm -f "../$1-$x.fail" && exit 77
+		test $r -eq 77 && rm -f "../$1-$x.fail" && exit 77
 		diff -u "$name.xx" "$name.right" >"../$1-$x.fail" && rm -f "$name.xx" "../$1-$x.fail"
 	)
 	case $? in