[struct tspec] (hexl_mode_trailer): Rename from `trailer.'

[struct tspec] (hexl_mode_trailer): Rename from `trailer.'
(dump_hexl_mode_trailer): Rename from dump_string_trailer.
Use fputs and putchar instead of trivial or %-less printfs.
(decode_one_format): Parenthesize each field_width assignment in an
argument list to make the side effect a little more apparent.
(write_block): Use fputs and putchar instead of trivial or %-less printfs.
(dump_strings): Cast string_min to off_t to avoid long-standing warning.