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

installer/src/installer.js

@@ -36,6 +36,7 @@ util.inherits(InstallerError, Error);
 InstallerError.INTERNAL_ERROR = 1;
 InstallerError.ALREADY_PROVISIONED = 2;
 
+// system until file has KillMode=control-group to bring down child processes
 function spawn(tag, cmd, args, callback) {
     assert.strictEqual(typeof tag, 'string');
     assert.strictEqual(typeof cmd, 'string');