/etc/init.d/grub-common uses file from initscripts without a dependency

In yakkety we are only a few away from dropping initscripts from the default install. When purging initscripts, the only failed service at boot is /etc/init.d/grub-common:

● grub-common.service - LSB: Record successful boot for GRUB
   Loaded: loaded (/etc/init.d/grub-common; bad; vendor preset: enabled)
   Active: failed (Result: exit-code) since Fr 2016-05-20 18:17:46 CEST; 23s ago
     Docs: man:systemd-sysv-generator(8)
  Process: 761 ExecStart=/etc/init.d/grub-common start (code=exited, status=2)

Mai 20 18:17:46 donald systemd[1]: Starting LSB: Record successful boot for GRUB...
Mai 20 18:17:46 donald grub-common[761]: /etc/init.d/grub-common: 17: .: Can't open /lib/init/vars.sh
Mai 20 18:17:46 donald systemd[1]: grub-common.service: Control process exited, code=exited status=2
Mai 20 18:17:46 donald systemd[1]: Failed to start LSB: Record successful boot for GRUB.
Mai 20 18:17:46 donald systemd[1]: grub-common.service: Unit entered failed state.
Mai 20 18:17:46 donald systemd[1]: grub-common.service: Failed with result 'exit-code'.

Indeed /etc/init.d/grub-common unconditionally sources /lib/init/vars.sh without depending on initscripts. Of course we don't actually want to add this dependency, but robustify the init.d script to get along without vars.sh. In addition it wouldn't hurt to create a systemd unit for this.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: grub-common 2.02~beta2-36ubuntu3
ProcVersionSignature: Ubuntu 4.4.0-23.41-generic 4.4.10
Uname: Linux 4.4.0-23-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: i3
Date: Fri May 20 18:19:26 2016
EcryptfsInUse: Yes
SourcePackage: grub2
UpgradeStatus: No upgrade log present (probably fresh install)