ed: add unicode support for the l (list) command

Use mbrtowc()/iswprint()/wcwidth() in put_tty_line() so that
the l command displays valid multibyte characters as-is instead
of escaping each byte as octal.

Column wrapping now correctly accounts for character display
width (including double-width CJK characters).

Invalid or incomplete UTF-8 sequences and non-printable
characters are still escaped as octal.

Differential Revision: https://reviews.freebsd.org/D55365