unix.Kill() does not produce an error for PID 0, -1. As a result, checking
process.Alive() would return "true" for both 0 and -1 on macOS (and previously
on Linux as well).
Let's shortcut these values to consider them "not alive", to prevent someone
trying to kill them.
A basic test was added to check the behavior.
Given that the intent of these functions is to handle single processes, this patch
also prevents 0 and negative values to be used.
From KILL(2): https://man7.org/linux/man-pages/man2/kill.2.html
If pid is positive, then signal sig is sent to the process with
the ID specified by pid.
If pid equals 0, then sig is sent to every process in the process
group of the calling process.
If pid equals -1, then sig is sent to every process for which the
calling process has permission to send signals, except for
process 1 (init), but see below.
If pid is less than -1, then sig is sent to every process in the
process group whose ID is -pid.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
735e250326