- Do stream mode for commands done by exec module, it seems important
in some cases (e.g. ensure requested stdin is fully written).
- For cases where the raw input reader knows the child process has been
reaped, set the childpid member to a sentinel value to indicate such
so we don't later think we should kill it or wait on it anymore.
- More error checking on dup2/close calls. Set sentinel values when
closing ends of pipes to prevent double closing a fd.
- Signal flag not set when raw input reader's child exits as a result
of a signal. Left out a test for this -- might be portability issues
(e.g. Ubuntu seems to do things different regarding the exit code and
also is printing "Killed" to stderr where other platforms don't).
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