fold: add the --characters option

fold: add the --characters option

* src/fold.c: Include mcel.h.
(count_bytes): Remove variable.
(counting_mode, last_character_width): New variables.
(shortopts, long_options): Add the option.
(adjust_column): If --characters is in used account for number of
characters instead of their width.
(fold_file): Use getline and iterate over the result with mcel functions
to handle multibyte characters.
(main): Check for the option.
* src/local.mk (src_fold_LDADD): Add $(LIBC32CONV), $(LIBUNISTRING), and
$(MBRTOWC_LIB).
* tests/fold/fold-characters.sh: New file.
* tests/fold/fold-spaces.sh: New file.
* tests/fold/fold-nbsp.sh: New file.
* tests/local.mk (all_tests): Add the tests.
* NEWS: Mention the new option.
* doc/coreutils.texi (fold invocation): Likewise.