/lib/systemd/system/opensmtpd.service uses wrong syntax `ExecStop=/usr/sbin/smtpctl stop`

[Norbert]

Steps to reproduce:
1. Install `opensmtpd` package on 18.04 LTS
2. Try to stop OpenSMTPd service with `sudo service opensmtpd stop`

Expected results:
* OpenSMTPd service is stopped

Actual results:
* OpenSMTPd service is not stopped
* the /lib/systemd/system/opensmtpd.service uses wrong syntax `ExecStop=/usr/sbin/smtpctl stop`
which really is not supported by `smtpctl`.

Notes:
1. First seen on AskUbuntu ( https://askubuntu.com/q/1166462/66509 )
2. it is known that this problem is fixed in newer releases ( see http://changelogs.ubuntu.com/changelogs/pool/universe/o/opensmtpd/opensmtpd_6.0.3p1-4/changelog )

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: opensmtpd 6.0.3p1-1build1
ProcVersionSignature: Ubuntu 4.15.0-54.58-generic 4.15.18
Uname: Linux 4.15.0-54-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.7
Architecture: amd64
CurrentDesktop: MATE
Date: Sun Aug 18 17:41:09 2019
InstallationDate: Installed on 2018-04-26 (478 days ago)
InstallationMedia: Ubuntu-MATE 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
SourcePackage: opensmtpd
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.