package/mender: Fix incorrectly named service file

Many of the mender CLI commands use systemctl commands to get information about
the daemon, such as the PID (IE: systemctl show -p MainPID mender-client).
As seen above, these commands expect the service file to be named
"mender-client" instead of "mender."

As such, in the current state, running a forced update check in the CLI will
result in the following error:
failed to force updateCheck: could not find the PID of the mender daemon.

Changing the name of mender.service to mender-client.service fixes the issue.

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>