Make the character table available to libroff so it can check the

Make the character table available to libroff so it can check the
validity of character escape names and warn about unknown ones.
This requires mchars_spec2cp() to report unknown names again.
Fortunately, that doesn't require changing the calling code because
according to groff, invalid character escapes should not produce
output anyway, and now that we warn about them, that's fine.