Stopping pihole-FTL process returns non-zero exit code

[mbiebl]

When running pihole-FTL in foreground mode (via -f), stopping the process via kill, will terminate the process, stop successfully, but then returns -1. This makes it hard to use pihole-FTL with a different service manager like runit or systemd, as they will mark the process as failed on stop.
To reproduce the issue, run

# runuser -u pihole /usr/bin/pihole-FTL -- -f
(run kill $(pid_of pihole-FTL) )
# echo $?
1

The logs show that pihole-FTL did shutdown properly:

# tail -n 3 /var/log/pihole-FTL.log 
[2019-02-19 20:00:52.430 535] Shutting down...
[2019-02-19 20:00:52.844 535] Finished final database update
[2019-02-19 20:00:52.846 535] ########## FTL terminated after 24204.6 ms! ##########

...

[DL6ER]

This is fixed since v4.3