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

[Ansgar Burchardt]

Control: retitle -1 init: Move "Essential: yes" from init to init-system-helpers
Control: tag -1 - patch
Control: forcemerge -1 823501

I would like "init" to be optional in Debian 9 for chroot environments
and some uses of containers.  A first step seems to be making "init" no
longer essential; it would nice nice if the priority could later be
downgraded as well (to "important") so that a minimal debootstrap will
not install it.

As maintainer scripts call `invoke-rc.d` and `update-rc.d`
unconditionally, the package providing these scripts should be made
essential (i.e. add "Essential: yes" to init-system-helpers).

So, suggestion to proceed:

 - Now:
   + "init" package: Remove "Essential: yes"
   + "init-system-helpers" package: Add "Essential: yes"
 - Later:
   + "init": Change priority from "required" to "important".

Do we miss anything to start with moving the "Essential" field?  I think
/bin/pidof was mentioned which is provided by sysvinit-utils; however
sysvinit-utils will continue to stay quasi-essential as util-linux has
  Depends: sysvinit-utils (>= 2.88dsf-59.1~)
and util-linux is an essential package.

And do we miss anything for the priority change after that (besides
confirming with d-boot@)?

Ansgar