client from deb package: commands check-update and send-inventory do not work

The bug affects both official Debian packages (see reported issue here) and our own .deb package build.

The underlying issue is quite clear: mender code assumes that the systemd service is "mender", so it fails on deb packages where the service is called "mender-client".

pi@raspberrypi:~$ mender -check-update
ERRO[0000] updateCheck: Failed to send 0 the mender process, pid: 0  module=main
pi@raspberrypi:~$ mender -send-inventory
ERRO[0000] updateCheck: Failed to send 0 the mender process, pid: 0  module=main

This bug will be N.A. in master and future releases after MEN-2984, but we still need a bug fix for 2.3.1 Mender release (client 2.2.x) and official Debian packages.