rootless: set systemd KillMode to mixed

Now `systemctl --user stop docker` completes just with in 1 or 2 seconds.

Fix issue 41944 ("Docker rootless does not exit properly if containers are running")

See systemd.kill(5) https://www.freedesktop.org/software/systemd/man/systemd.kill.html

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

contrib/dockerd-rootless-setuptool.sh

@@ -307,6 +307,7 @@ install_systemd() {
 			TasksMax=infinity
 			Delegate=yes
 			Type=simple
+			KillMode=mixed
 
 			[Install]
 			WantedBy=default.target