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...
be994c2640