Add command line option to skip pages that contain font data

If a page contains font data, the script would abort, unless -f was given,
in which case it would use pdftoppm to rasterize the font into a bitmap
and then attempt to OCR it. -f is almost certainly not what users want
unless they want to debug OCR or something.

If a PDF already has fonts it either was OCR'd already, or it is
a composite file containing, for example, some scanned documents appended
to a text report.  In the latter case, this -s option provides OCR on
pages that don't have it without changing those that do, and if a PDF
was completely OCRed it will be converted to PDF/A.  In batch jobs with
a mix of OCR and non-OCR the implicit conversion to PDF/A is also useful.