core: don't use the unified hierarchy for the systemd cgroup yet (#4628)

Too many things don't get along with the unified hierarchy yet:

 * opencontainers/runc#1175
 * moby/moby#28109
 * lxc/lxc#1280

So revert the default to the legacy hierarchy for now. Developers of the above
software can opt into the unified hierarchy with
"systemd.legacy_systemd_cgroup_controller=0".

src/basic/cgroup-util.c

@@ -2423,10 +2423,10 @@ bool cg_is_unified_systemd_controller_wanted(void) {
 
                 r = get_proc_cmdline_key("systemd.legacy_systemd_cgroup_controller=", &value);
                 if (r < 0)
-                        return true;
+                        return false;
 
                 if (r == 0)
-                        wanted = true;
+                        wanted = false;
                 else
                         wanted = parse_boolean(value) <= 0;
         }