email-print-mime-structure: use only BOX DRAWINGS

[Daniel Kahn Gillmor]

Hi!  This brief series proposes to change the output of
email-print-mime-structure to use only the BOX DRAWINGS characters
from unicode, rather than using more unusual characters.

I've been using these MIME diagrams for Internet Drafts within the
IETF, and they're struggling with their PDF output, which is
constrained to a more limited set of glyphs due to a minimal set of
permitted fonts which themselves only have limited coverage of symbols
in a fixed-width font.  In particular, this is motivated by the
discussion at https://github.com/ietf-tools/xml2rfc/issues/1245

Regards,

        --dkg