Use invoke-rc.d in maintainer scripts

Upstart configurations and sysv init scripts should be started and stopped
using invoke-rc.d in maintainer scripts.

This ensures the correct behaviour across both Debian (sysv) and Ubuntu (upstart)
and in restricted environments such as schroot where start/stop of services from
maintainer scripts is normally disabled.

Signed-off-by: James Page <james.page@ubuntu.com>
Merge pull request #304 from javacruft/wip-invoke-rc.d

Use invoke-rc.d in maintainer scripts