battery-check: several follow-ups

battery-check: several follow-ups

Follow-ups for e3d4148d50909119c4e9327e6ad96d3ca7f4661a.

- 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...