Ticket #2713: mcedit: respect multibyte characters in paragraph format.

Treate the option_word_wrap_line_length option as the line length in
screen cells not in bytes.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Merge branch '2713_format_paragraph_multibyte'

* 2713_format_paragraph_multibyte:
  Ticket #2713: mcedit: respect multibyte characters in paragraph format.