Log Message:
htmldoc: update to 1.9.8.

ChangeLog:

- Added support for a `HTMLDOC.filename` META keyword that controls the filename
  reported in CGI mode; the default remains "htmldoc.pdf" (Issue #367)
- Fixed a paragraph formatting issue with large inline images (Issue #369)
- Fixed a buffer underflow issue (Issue #370)
- Fixed PDF page numbers (Issue #371)
- Added support for a new `L` header/footer format (`$LETTERHEAD`), which
  inserts a letterhead image at its full size (Issue #372, Issue #373,
  Issue #375)
- Updated the build documentation (Issue #374)