Print out header fields as PDF metadata; Add simple markup->string function

Print out header fields as PDF metadata; Add simple markup->string function

-) Create DOCINFO pdfmark, with the metadata fields extracted
   from the \header block

-) Add scheme function markup->string that extracts and returns only the
   string part of a markup. As all header fields are possibly markups,
   we need to call this function on the header fields and use only
   the string representation (formatting and possibly some information
   lost!) as metadata.
   This function is very simply (for all known text markup functions,
   it extracts the text and ignores the formatting, all other markup
   functions are entirely ignored), so it might not work perfectly
   for complex markups.

-) Add possibility to override a header field with the same name, but
   a pdf prefix, to force a particular metadata string, which is not
   displayed in the pdf. E.g. if the \header block contains
       pdftitle = "Title for pdf file"
   then that pdftitle header field will be used for the document title
   in the PDF metadata instead of the title header field.