systemd-run-i3: Add ExitType=cgroup to user units

systemd v250 added ExitType= which allows systemd to wait for all
processes in the cgroup to exit before terminating the unit, instead of
only the main pid. This has many uses, but for desktop applications in
particular it allows self-restarting (forking) applications to correctly
fork/restart and other background processes that may have been spawned
to continue when their forked parent inevitably exits first (like ssh
processes inside terminals).

Signed-off-by: Joe Groocock <me@frebib.net>

scripts/systemd-run-i3

@@ -1,5 +1,6 @@
 #!/bin/bash -e
 
+slice=app
 rand=$(tr -dc 'a-f0-9' < /dev/urandom | head -c 8)
 
 args=()
@@ -16,14 +17,11 @@ do case "$opt" in
 done
 shift $((OPTIND-1))
 
-if [ -z "$slice" ]; then
-    slice=app
-fi
-
 exec systemd-run \
     --user \
     --collect \
     --slice="$slice" \
     --unit="$slice-$1-$rand" \
     --property=BindsTo=graphical-session.target \
-    ${args[@]} -- "$@"
+    --property=ExitType=cgroup \
+    "${args[@]}" -- "$@"