v5: merge v6 to v5, version bump to 5.4.*, reason adds structure checksisu_5.4.0

* ao, document structure check, stop processing on major error
  (with error message & text at location of failure)
  [reason for version bump the addition of structure check]
* utils, add ok code marker
* ao, heading with no ocn, distinguish ~# from -#
  * ~# is general & means no ocn (for any object to which it is applied)
  * -# is relevant only for 1~ dummy headings & instructs that they should be
    removed from output where possible
  * applied so far to pdf, odt & plaintext
* ao_images, reduce warnings when ruby RMagic absent as program used directly
  * reduced dependency on ruby RMagic library (as some time way back had
    issues)
* ao, document markup structure check, skip processing file on major error
  * with error message & text at location of failure
* texpdf, urls in creator cause breakage
* texpdf, mailto markup links set for normal text objects
* texpdf, '&' in heading breaks toc (now removed from toc (not heading)), bug
  * revisit, bug
* texpdf, for urls switch to sans serif (small fontsize)
  * instead of typewriter, latex default
* texpdf, pdf colored hyperlinks configurable
  * --pdf-hyperlinks-color
    --pdf-hyperlinks-no-color or --pdf-hyperlinks-monochrome
  * ['default']['pdf_hyperlinks']='color'
    (other options switch hyperlink color off 'no-color' 'color-off'
    'monochrome')
* texpdf, pdf default font size configurable (cli & sisurc.yml) (no fractions)
  * --pdf-fontsize-12 --pdf-fontsize-8
  * default:
    texpdf_fontsize: 12
* texpdf, headings and table of contents representation
  * fixes 1~ and 2~ result in the same formatting
    1~ 2~ & 3~ now differentiated, see discussion in sisu.org under #744383
  * fixes :A smaller formatted than :B
    A~ B~ C~ now the same size, see discussion in sisu.org under #744383
  * :B and :C result in the same formatting, issue explained see sisu.org
* texpdf, (internal coding) fontface, rename texpdf_font texpdf_fontface
* texpdf, (internal coding) use symbols to identify page orientation
* param, metadata rights, line-breaks instead of semicolons separating rights
  * fixes remove trailing semicolon after :copyright:
* digests sha512 option implemented
  * options sha512 sha256 md5
  * command line --sha512
  * rcconf.yml ['defsault']['digest'] = sha512
* xml object citation numbering (docbook fictionbook)