package/mender: fix sysv startup script

Mender is a service explicitly written for systemd and so it doesn't
fork on background, doesn't redirect outputs and doesn't create a pid
file by itself.
To make the service running correctly is therefore necessary to use the
-m switch of start-stop-daemon to create the pid file and -b option to
send the process to background.
Logging is preserved because the service will log anyway on syslog.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>