showart: refactoring ahead!

We now support embedded font hints in SAUCE records! So if no encoding hint
is passed to the showart() function, it will:

  1. see if there is a font in the SAUCE record
  2. use the configured system.art_utf8_codec encoding
  3. fall back to CP437 encoding

This should greatly enhance the experience for UTF-8 enabled BBS users, as
they will see a piece rendered more closely to its original form (depending
on how well their font approximates the original one, of course).