fix exit status

[TimWhiting]

Fix exit status on unix for system call. The returned integer is not directly interpretable as an exit status.

Fixes: #797

Unfortunately, unix does this to require the user to check for other abnormal termination (due to signal / stopped process).

Not sure how you want to report that?

• Add a tagged C union in kklib, and convert it to a Koka Error type in std/os/process?
• Assignable parameters in kklib, ditto for std/os/proccess?
• Something else?

My opinion is to do the simple fix now (below), with the plan of re-addressing this when we get to real process integration with libuv.

@daanx

[timbertson]

I'm surprised stopped is a different branch, isn't that just another signal?

FWIW in my WIP libuv process branch, I went with an ADT for Exit-status(int) and Exit-signal(int). I don't expect most people would need to inspect this directly, they just want to check for success or not.

Edit: Ah, looks like stopped would not happen here, according to the docs:

WIFSTOPPED(wstatus)
returns true if the child process was stopped by delivery
of a signal; this is possible only if the call was done
using WUNTRACED or when the child is being traced (see
ptrace(2)).

[timbertson]

FWIW haskell returns the negative of this value to distinguish it from an exit status, based on my various failed runs which return -11 (SIGSEGV) 🤦