core: add DelegateSubgroup= setting

This implements a minimal subset of #24961, but in a lot more
restrictive way: we only allow one level of subcgroup (as that's enough
to address the no-processes in inner cgroups rule), and does not change
anything about threaded cgroup logic or similar, or make any of this new
behaviour mandatory.

All this does is this: all non-control processes we invoke for a unit
we'll invoke in a subgroup by the specified name.

We'll later port all our current services that use cgroup delegation
over to this, i.e. user@.service, systemd-nspawn@.service and
systemd-udevd.service.

src/shared/bus-unit-util.c

@@ -461,7 +461,8 @@ static int bus_append_cgroup_property(sd_bus_message *m, const char *field, cons
                               "ManagedOOMSwap",
                               "ManagedOOMMemoryPressure",
                               "ManagedOOMPreference",
-                              "MemoryPressureWatch"))
+                              "MemoryPressureWatch",
+                              "DelegateSubgroup"))
                 return bus_append_string(m, field, eq);
 
         if (STR_IN_SET(field, "ManagedOOMMemoryPressureLimit")) {