[unix] `Output::Killed` in results may be inaccurate

[Silic0nS0ldier]

libc's rather poorly named kill(...) function returns when the specific signal is delivered. This means;

• For SIGTERM, kill(...) may return before the target process has terminated. It may even ignore the signal.
• For SIGKILL, the operation is handled by the kernel so generally by the time kill(...) returns the target will be terminated. However that isn't certain.

Both cases also may be rejected with EPERM (permission error).

Unfortunately checking process liveliness (e.g. with getpid on macOS or /proc/ on Linux) can't provide clarity here either, since process IDs are reused (not an issue on Windows from memory). Not to mention liveliness checks would need to contend with SIGTERM potentially doing nothing and both taking an undefined length of time to take effect.

The end result is Output::Killed is more of a Output::MaybeKilled.