Configure systemd to send SIGQUIT on stop (#1702)

dist/init/linux-systemd/caddy.service

@@ -20,6 +20,11 @@ Environment=CADDYPATH=/etc/ssl/caddy
 ExecStart=/usr/local/bin/caddy -log stdout -agree=true -conf=/etc/caddy/Caddyfile -root=/var/tmp
 ExecReload=/bin/kill -USR1 $MAINPID
 
+; Use graceful shutdown with a reasonable timeout
+KillMode=mixed
+KillSignal=SIGQUIT
+TimeoutStopSec=5s
+
 ; Limit the number of file descriptors; see `man systemd.exec` for more limit settings.
 LimitNOFILE=1048576
 ; Unmodified caddy is not expected to use more than that.