hush: fix a bug with "stdio rewind on exit". Closes 9186

With FEATURE_SH_STANDALONE=y, 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.

`cmd` and arithmetic also need the fix for this, even without
FEATURE_SH_STANDALONE.

Fix this by _exit()ing instead.

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

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