(PUP-3549) Fix unsafe stopping of puppet agent service in Debian init script.

Pass process name to start-stop-daemon in Debian init script, so that it
is safer to stop or reload the puppet service. After stop, make sure the
PID file is removed for extra safety.
Merge pull request #3239 from tomasz-kuzemko/pup-3549-debian-init

(PUP-3549) Fix unsafe stopping of puppet agent service in Debian init script.