basic/{time,format}-util: warn when format result is unused

Now that anonymous buffers are used in almost all cases, code which
does not use the return value is usually broken.

src/basic/format-util.h

@@ -74,8 +74,9 @@ typedef enum {
 
 #define FORMAT_BYTES_MAX 16U
 
-char *format_bytes_full(char *buf, size_t l, uint64_t t, FormatBytesFlag flag);
+char *format_bytes_full(char *buf, size_t l, uint64_t t, FormatBytesFlag flag) _warn_unused_result_;
 
+_warn_unused_result_
 static inline char *format_bytes(char *buf, size_t l, uint64_t t) {
         return format_bytes_full(buf, l, t, FORMAT_BYTES_USE_IEC | FORMAT_BYTES_BELOW_POINT | FORMAT_BYTES_TRAILING_B);
 }