dbus-cgroup: Make sure we overwrite cpuset properties in drop-in

The DBUS property setter overwrites the value of the property but
writes a drop-in that extends the value. Let's make sure the drop-in
overwrites the property value as well by assigning the empty string
first.

(cherry picked from commit 1dbccd6)

src/core/dbus-cgroup.c

@@ -1223,7 +1223,7 @@ int bus_cgroup_set_property(
                         new_set = (CPUSet) {};
 
                         unit_invalidate_cgroup(u, CGROUP_MASK_CPUSET);
-                        unit_write_settingf(u, flags, name, "%s=%s", name, setstr);
+                        unit_write_settingf(u, flags, name, "%s=\n%s=%s", name, name, setstr);
                 }
 
                 return 1;