fix: Prefer os.Process.Signal to syscall.Kill

This helps protect us from a potential (but extremely rare) race
condition where

1. the process exits terminates
2. its PID gets reaped in the Wait call
3. a new process with the same PID gets spawned

before our signal actually gets sent.

This uses the higher-level os.Process API directly, which has some
synchronization in-place to prevent Wait and Signal from racing.

src/command/stopper_unix.go

@@ -28,7 +28,7 @@ func (c *CmdWrapper) Stop(sig int, parentOnly bool) error {
 		Msg("Stop Unix process.")
 
 	if parentOnly {
-		return syscall.Kill(c.Pid(), syscall.Signal(sig))
+		return c.cmd.Process.Signal(syscall.Signal(sig))
 	}
 
 	pgid, err := syscall.Getpgid(c.Pid())