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

baseimage/initializeBaseUbuntuImage.sh

@@ -227,7 +227,8 @@ Description=Cloudron Installer
 Type=idle
 ExecStart="${INSTALLER_SOURCE_DIR}/src/server.js"
 Environment="DEBUG=installer*,connect-lastmile" ${provisionEnv}
-KillMode=process
+; kill any child (installer.sh, retire.sh) as well
+KillMode=control-group
 Restart=on-failure
 
 [Install]