Make sure to relay any remaining stderr from the process after it has
shut down, rather than closing stderr just before shutdown. This avoids
a situation where the process is still running and tries to write to
stderr, getting a SIGPIPE. And, it ensures that no stderr output is
lost.
This may fix a problem encountered by datalad on windows, where it hangs
during the external special remote shutdown.
Before commit a49d300545, it closed stdin
and stdout, but left stderr open, and never killed the stderr waiter
thread, which presumably exited on its own. For async exception
safety, do need to at make sure that thread gets waited on, as that
commit does, but it introduced this problem.
Note that, the process's stdout is closed before waiting on it. It's too
late for anything it writes to stdout to be processed, and since we're
not going to consume any such writes, this avoids the process getting
blocked writing to stdout due to us not reading what it's buffered. This
does mean that if the process writes to stdout too late, it will get a
SIGPIPE. (This was already the case before the above-mentioned commit.)
In practice, I think only the protocol's ERROR is allowed to be
sent at a point where this could happen.
