digraphs: alternate glyph string in include

Allow the digraph include to specify string overrides in cases where a
glyph should not be shown literally.  These are:

- Combining characters: prepend a placeholder.
  Dead chars are invalid on their own in fact.
- ASCII control characters: substitude display symbols at U+24xx.
  Though browsers usually show a character placeholder, it's not very
  nice to send control chars directly.
- Other control characters: show the replacement character U+FFFD.
  Actually with some (Linux) fonts, the anonymous code point fallback is
  more descriptive, but better to be on the safe side (they still have
  semantic value after all).