core: rework the Delegate= unit file setting to take a list of controller names

Previously it was not possible to select which controllers to enable for
a unit where Delegate=yes was set, as all controllers were enabled. With
this change, this is made configurable, and thus delegation units can
pick specifically what they want to manage themselves, and what they
don't care about.

src/shared/bus-unit-util.c

@@ -183,6 +183,51 @@ int bus_append_unit_property_assignment(sd_bus_message *m, const char *assignmen
 
                 r = sd_bus_message_append(m, "sv", field, "t", bytes);
                 goto finish;
+
+        } else if (streq(field, "Delegate")) {
+
+                r = parse_boolean(eq);
+                if (r < 0) {
+                        const char *p = eq;
+
+                        r = sd_bus_message_append(m, "s", "DelegateControllers");
+                        if (r < 0)
+                                goto finish;
+
+                        r = sd_bus_message_open_container(m, 'v', "as");
+                        if (r < 0)
+                                goto finish;
+
+                        r = sd_bus_message_open_container(m, 'a', "s");
+                        if (r < 0)
+                                goto finish;
+
+                        for (;;) {
+                                _cleanup_free_ char *word = NULL;
+
+                                r = extract_first_word(&p, &word, NULL, EXTRACT_QUOTES);
+                                if (r == 0)
+                                        break;
+                                if (r == -ENOMEM)
+                                        return log_oom();
+                                if (r < 0)
+                                        return log_error_errno(r, "Invalid syntax: %s", eq);
+
+                                r = sd_bus_message_append(m, "s", word);
+                                if (r < 0)
+                                        goto finish;
+                        }
+
+                        r = sd_bus_message_close_container(m);
+                        if (r < 0)
+                                goto finish;
+
+                        r = sd_bus_message_close_container(m);
+                } else
+                        r = sd_bus_message_append(m, "sv", "Delegate", "b", r);
+
+                goto finish;
+
         } else if (streq(field, "TasksMax")) {
                 uint64_t t;
 
@@ -238,7 +283,7 @@ int bus_append_unit_property_assignment(sd_bus_message *m, const char *assignmen
                               "TasksAccounting", "IPAccounting", "SendSIGHUP", "SendSIGKILL", "WakeSystem",
                               "DefaultDependencies", "IgnoreSIGPIPE", "TTYVHangup", "TTYReset", "TTYVTDisallocate",
                               "RemainAfterExit", "PrivateTmp", "PrivateDevices", "PrivateNetwork", "PrivateUsers",
-                              "NoNewPrivileges", "SyslogLevelPrefix", "Delegate", "RemainAfterElapse",
+                              "NoNewPrivileges", "SyslogLevelPrefix", "RemainAfterElapse",
                               "MemoryDenyWriteExecute", "RestrictRealtime", "DynamicUser", "RemoveIPC",
                               "ProtectKernelTunables", "ProtectKernelModules", "ProtectControlGroups", "MountAPIVFS",
                               "CPUSchedulingResetOnFork", "LockPersonality")) {