Somewhat fixing sysvinit and casper legacy support, slightly improved patch...

Somewhat fixing sysvinit and casper legacy support, slightly improved patch from Rui Miguel P. Bernardo <rui.bernardo.pt@gmail.com> (Closes: #772522, #772651).

This however will still not work properly building upstart based
images but they have been broken/unsupported anyway since a long
time.