core/service: actually allow to "hurry up" auto restarts

unit_start() advertises that start requests don't get suppressed,
so that it could be used to manually speed up auto restarts.
However, service_start() so far rejected this, stating that
clients should issue restart request in order to trigger
BindsTo=/OnFailure=.

That seems to be a red herring though, because for a long time
the service states between auto-restarts were buggy (#27594).
With the introduction of RestartMode=direct, the behavior
is sane again and customizable, hence I see no reason to refuse
this anymore. Whether those deps are triggered solely depends
on RestartMode= now.

Plus, filter out some intermediate states that should never
be seen in service_start().

Fixes #33890

test/units/TEST-03-JOBS.sh

@@ -1,5 +1,7 @@
 #!/usr/bin/env bash
 # SPDX-License-Identifier: LGPL-2.1-or-later
+# shellcheck disable=SC2016
+
 set -eux
 set -o pipefail
 
@@ -166,4 +168,30 @@ assert_rc 3 systemctl --quiet is-active succeeds-on-restart.target
 systemctl start fails-on-restart.target || :
 assert_rc 3 systemctl --quiet is-active fails-on-restart.target
 
+# Test shortcutting auto restart
+
+export UNIT_NAME="TEST-03-JOBS-shortcut-restart.service"
+TMP_FILE="/tmp/test-03-shortcut-restart-test$RANDOM"
+
+cat >"/run/systemd/system/$UNIT_NAME" <<EOF
+[Service]
+Type=oneshot
+ExecStart=rm -v "$TMP_FILE"
+Restart=on-failure
+RestartSec=1d
+RemainAfterExit=yes
+EOF
+
+(! systemctl start "$UNIT_NAME")
+timeout 10 bash -c 'while [[ "$(systemctl show "$UNIT_NAME" -P SubState)" != "auto-restart" ]]; do sleep .5; done'
+touch "$TMP_FILE"
+assert_eq "$(systemctl show "$UNIT_NAME" -P SubState)" "auto-restart"
+
+timeout 30 systemctl start "$UNIT_NAME"
+systemctl --quiet is-active "$UNIT_NAME"
+assert_eq "$(systemctl show "$UNIT_NAME" -P NRestarts)" "1"
+[[ ! -f "$TMP_FILE" ]]
+
+rm /run/systemd/system/"$UNIT_NAME"
+
 touch /testok