cgroup: Imply systemd.unified_cgroup_hierarchy=1 on cgroup_no_v1=all

cgroup_no_v1=all doesn't make a whole lot of sense with legacy hierarchy
(where we use v1 hierarchy for everything), or hybrid hierarchy (where
we still use v1 hierarchy for resource control).

Right now we have to tell people to add both cgroup_no_v1=all and
systemd.unified_cgroup_hierarchy=1 to get the desired behaviour,
however in reality it's hard to imagine any situation where someone
passes cgroup_no_v1=all but *doesn't* want to use the unified cgroup
hierarchy.

Make it so that cgroup_no_v1=all produces intuitive behaviour in systemd
by default, although it can still be disabled by passing
systemd.unified_cgroup_hierarchy=0 explicitly.

src/test/test-cgroup-util.c

@@ -369,6 +369,17 @@ static void test_is_wanted(void) {
                          "systemd.unified_cgroup_hierarchy=0 "
                          "systemd.legacy_systemd_cgroup_controller=0", 1) >= 0);
         test_is_wanted_print(false);
+
+        /* cgroup_no_v1=all implies unified cgroup hierarchy, unless otherwise
+         * explicitly specified. */
+        assert_se(setenv("SYSTEMD_PROC_CMDLINE",
+                         "cgroup_no_v1=all", 1) >= 0);
+        test_is_wanted_print(false);
+
+        assert_se(setenv("SYSTEMD_PROC_CMDLINE",
+                         "cgroup_no_v1=all "
+                         "systemd.unified_cgroup_hierarchy=0", 1) >= 0);
+        test_is_wanted_print(false);
 }
 
 static void test_cg_tests(void) {