Bug#756023: init: Move "Essential: yes" from init to init-system-helpers

[Ansgar Burchardt]

On Thu, 2016-05-19 at 18:27 +0200, Michael Biebl wrote:
> Could we add the Important: yes field to "init"? Having that makes a
> lot of sense to me.

To init or to its dependencies (systemd-sysv, sysvinit-core)?  The
latter would not only warn when uninstalling all init systems, but also
when switching between them (perhaps not by intention).

Ansgar