This does two things.
Important one: it reverts #19193, which lead to our whole process
handling breaking. When the `proxy` process was killed, it apparently
didn't close the stdout/stderr anymore, which meant we would not detect
when it died. (Watching its `status()` in the io loop also didn't work!)
We should figure out how to keep our process handling working before we
make this change in #19193, which sounds reasonable.
Second, less important thing: I think we should treat the process being
killed from a signal as non-zero, as an error.
Release Notes:
- N/A
e3c6ba4bd7