procutil: drop unused 'newlines' option from popen*() (API)
It's unlikely for us to use the universal_newlines option.
procutil: make explainexit() simply return a message (API)
Almost all callers want it.
procutil: do not convert return code of signal exit to positive number (API)
The docstring states that "codes from kill are negative", and it doesn't
make sense to make exit/signal code ambiguous.
.. api::
``hook.hook()`` and ``hook.runhooks()`` may return a negative integer
to denote that the process was killed by signal.
procutil: fix error message of tempfile filter
First, we need to use procutil.system() to get an exit code compatible with
explainexit(). Second, explainexit() returns (msg, code) tuple.
procutil: unify platform.explainexit()
Since
4368f582c806 "use subprocess instead of os.system", posix.explainexit()
is the superset of Windows implementation.