Make it easier to port sysvinit jobs that use should-start to upstart

[Stéphane Graber]

During Debconf2013 it was mentioned that a fair amonut of sysvinit jobs using the LSB headers use the Should-Start function which essentially means "if that job exists, then it needs to start first".

Upstart currently doesn't allow that but it should be easy to have it do so.

One thought is adding a fairly simple helper, possibly even just a shell script which essentially does:
 - Scan through all the available jobs, looking for those looking for a "missing-job" event
 - For each of those, look into /etc/init for a job of that name, if it doesn't exist, emit "missing-job"

That way, if you take a software that needs rsyslog and should start after dbus if present, we'd get:
start on started rsyslog and (started dbus or missing-job dbus)