battery-check: several follow-ups

Follow-ups for e3d4148d50.

- add reference to initrd-battery-check.service in man page, and move
  its section from 1 to 8,
- add link to man page in help message,
- introduce ERRNO_IS_NO_PLYMOUTH(),
- propagate error in battery_check_send_plymouth_message(),
- rename battery_check_send_plymouth_message() -> plymouth_send_message(),
- return earlier when the first battery level check passed to reduce
  indentation,
- fix potential use of invalid fd on battery restored,
- do not use emoji for /dev/console,
- add simple test (mostly for coverity),

etc, etc...

test/units/testsuite-74.battery-check.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -eux
+set -o pipefail
+
+systemd-battery-check --help
+systemd-battery-check --version
+
+systemd-battery-check || :