Bug 18250: Force startup order in postinst script

Bug 18250: Force startup order in postinst script

This is a workaround meant to correct upgrading installs.
We force the order by disable/enable.

Test plan:
[1] When upgrading an existing install, check that the startup order
    after the upgrade has been corrected in /etc/rcX.d (say X=5).

Signed-off-by: Mirko Tietgen <mirko@abunchofthings.net>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>