MINOR: systemd: consider exit status 143 as successful

The master process will exit with the status of the last worker. When
the worker is killed with SIGTERM, it is expected to get 143 as an
exit status. Therefore, we consider this exit status as normal from a
systemd point of view. If it happens when not stopping, the systemd
unit is configured to always restart, so it has no adverse effect.

This has mostly a cosmetic effect. Without the patch, stopping HAProxy
leads to the following status:

    ● haproxy.service - HAProxy Load Balancer
       Loaded: loaded (/lib/systemd/system/haproxy.service; disabled; vendor preset: enabled)
       Active: failed (Result: exit-code) since Fri 2018-06-22 20:35:42 CEST; 8min ago
         Docs: man:haproxy(1)
               file:/usr/share/doc/haproxy/configuration.txt.gz
      Process: 32715 ExecStart=/usr/sbin/haproxy -Ws -f $CONFIG -p $PIDFILE $EXTRAOPTS (code=exited, status=143)
      Process: 32714 ExecStartPre=/usr/sbin/haproxy -f $CONFIG -c -q $EXTRAOPTS (code=exited, status=0/SUCCESS)
     Main PID: 32715 (code=exited, status=143)

After the patch:

    ● haproxy.service - HAProxy Load Balancer
       Loaded: loaded (/lib/systemd/system/haproxy.service; disabled; vendor preset: enabled)
       Active: inactive (dead)
         Docs: man:haproxy(1)
               file:/usr/share/doc/haproxy/configuration.txt.gz

contrib/systemd/haproxy.service.in

@@ -10,6 +10,7 @@ ExecReload=@SBINDIR@/haproxy -f $CONFIG -c -q
 ExecReload=/bin/kill -USR2 $MAINPID
 KillMode=mixed
 Restart=always
+SuccessExitStatus=143
 Type=notify
 
 # The following lines leverage SystemD's sandboxing options to provide