Kill child processes

On Unix, child processes are not killed when parent dies.

Each process is part of a process group (pgid). When pgid == pid,
it is the process group leader.

node creates child processes with the parent as the group leader
(detached = false).

You can send a signal to entire group using kill(-pgid), as in,
negative value in argument. Systemd can be made to do this by
setting the KillMode=control-group.

Unrelated: Process groups reside inside session groups. Each session
group has a controlling terminal. Only one process in the session
group has access to the terminal. Process group is basically like
a bash pipeline. A session group is the entire login session with only
one process having terminal access at a time.

Fixes #543

src/taskmanager.js

@@ -92,6 +92,7 @@ function startAppTask(appId) {
         return;
     }
 
+    // when parent process dies, apptask processes are killed because KillMode=control-group in systemd unit file
     gActiveTasks[appId] = child_process.fork(__dirname + '/apptask.js', [ appId ]);
 
     var pid = gActiveTasks[appId].pid;