core/unit: do not use unit path cache in unit_need_daemon_reload()

When unit_need_daemon_reload() calls unit_find_dropin_paths() to check
for new drop-in configs, the manager's unit path cache is used to limit
which directories are considered. If a new drop-in directory is created,
it may not be in the unit path cache, and hence unit_need_daemon_reload()
may return false, despite a new drop-in being present. However, if a
unit path cache is not given to unit_file_find_dropin_paths() at all,
then it behaves as if the target path was found in the unit path cache.

So, to fix this, adapt unit_find_dropin_paths() to take a boolean
argument indicating whether or not to pass along the unit path cache.
Set this to false in unit_need_daemon_reload().

Fixes #31752

test/units/TEST-07-PID1.issue-31752.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# -*- mode: shell-script; indent-tabs-mode: nil; sh-basic-offset: 4; -*-
+# ex: ts=8 sw=4 sts=4 et filetype=sh
+
+set -eux
+set -o pipefail
+
+# shellcheck source=test/units/util.sh
+. "$(dirname "$0")"/util.sh
+
+# Make sure NeedDaemonReload= considers newly created drop-ins.
+# Issue: https://github.com/systemd/systemd/issues/31752
+
+UNIT=test-issue-31752.service
+
+cleanup() {
+    rm -rf /run/systemd/system/"$UNIT" /run/systemd/system/"$UNIT".d
+    systemctl daemon-reload
+}
+
+trap cleanup EXIT
+
+cat > /run/systemd/system/"$UNIT" <<EOF
+[Service]
+ExecStart=/usr/bin/true
+RemainAfterExit=yes
+EOF
+
+systemctl daemon-reload
+systemctl start "$UNIT"
+assert_eq "$(systemctl show -P NeedDaemonReload "$UNIT")" no
+
+mkdir /run/systemd/system/"$UNIT".d
+cat > /run/systemd/system/"$UNIT".d/desc.conf <<EOF
+[Unit]
+Description=Test NeedDaemonReload status after creating drop-in
+EOF
+
+assert_eq "$(systemctl show -P NeedDaemonReload "$UNIT")" yes
+
+rm /run/systemd/system/"$UNIT".d/desc.conf
+
+assert_eq "$(systemctl show -P NeedDaemonReload "$UNIT")" no