ap: track critical child services and die if any of them dies

Critical child services include dhclients, vtunds and hostapds. We could
try to restart dead services, but that would make the logic way too
complex for now. Now we simply die and leave the restart decision to
higher layers (e.g. init can restart the whole puavo-wlanap when it
dies).

There are plans to rewrite the whole puavo-wlan project with a more
robust proramming language. Critical service restarting will be
revisited then.