Change the unix ExitCode encoding again! Now simply -signum

So rather than a bitfield encoding, covering normal exit codes, signals
and core dumps, we now simply use negative numbers for signal exit
codes. We thus do not report core dumps by this mechanism.

This encoding is a lot simpler to explain and use. It also happens to be
the same encoding as Python's subprocess package uses.

Anyone who cares about core dumps can still use the unix package.