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...

src/basic/glyph-util.h

@@ -50,18 +50,14 @@ typedef enum SpecialGlyph {
         _SPECIAL_GLYPH_INVALID = -EINVAL,
 } SpecialGlyph;
 
-const char *special_glyph_full(SpecialGlyph code, bool force_utf) _const_;
-
 bool emoji_enabled(void);
 
+const char *special_glyph_full(SpecialGlyph code, bool force_utf) _const_;
+
 static inline const char *special_glyph(SpecialGlyph code) {
         return special_glyph_full(code, false);
 }
 
-static inline const char *special_glyph_force_utf(SpecialGlyph code) {
-        return special_glyph_full(code, true);
-}
-
 static inline const char *special_glyph_check_mark(bool b) {
         return b ? special_glyph(SPECIAL_GLYPH_CHECK_MARK) : special_glyph(SPECIAL_GLYPH_CROSS_MARK);
 }