scope: allow unprivileged delegation on scopes

Previously it was possible to set delegate property for scope, but you
were not able to allow unprivileged process to manage the scope's cgroup
hierarchy. This is useful when launching manager process that  will run
unprivileged but is supposed to manage its own (scope) sub-hierarchy.

Fixes #21683

test/units/testsuite-19.sh

@@ -3,6 +3,16 @@
 set -eux
 set -o pipefail
 
+test_scope_unpriv_delegation() {
+    useradd test ||:
+    trap "userdel -r test" RETURN
+
+    systemd-run --uid=test -p User=test -p Delegate=yes --slice workload.slice --unit workload0.scope --scope \
+            test -w /sys/fs/cgroup/workload.slice/workload0.scope -a \
+            -w /sys/fs/cgroup/workload.slice/workload0.scope/cgroup.procs -a \
+            -w /sys/fs/cgroup/workload.slice/workload0.scope/cgroup.subtree_control
+}
+
 if grep -q cgroup2 /proc/filesystems ; then
     systemd-run --wait --unit=test0.service -p "DynamicUser=1" -p "Delegate=" \
                 test -w /sys/fs/cgroup/system.slice/test0.service/ -a \
@@ -31,6 +41,10 @@ if grep -q cgroup2 /proc/filesystems ; then
 
     # And now check again, "io" should have vanished
     grep -qv io /sys/fs/cgroup/system.slice/cgroup.controllers
+
+    # Check that unprivileged delegation works for scopes
+    test_scope_unpriv_delegation
+
 else
     echo "Skipping TEST-19-DELEGATE, as the kernel doesn't actually support cgroup v2" >&2
 fi