(#5908) Add support for new update-rc.d disable API

Added support for the new disable API to update-rc.d and
added spec tests to check this functionality. This change was made
because in versions of sysv-rc >= 2.88, 'update-rc.d stop' is broken and
actually enables the service.

We only changed the disable case as the enable case still works on
systems which use sysv-rc 2.88 or greater (atm, only Debian Lenny). We
wanted to change as little as possible because update-rc.d prints a
message stating that the new enable/disable API is unstable and may
change in the future.

Paired-with:Matt Robinson, Jacob Helwig