Update `help' functionality:
    
        - Rename variable prefix from MANUAL_CHAN to MANUAL_CHAP.
    
        - Rename variable prefix from MANUAL_SECN to MANUAL_SECT.
    
        - Update the way documentation entry is retrived from
          `centos-art.sh' script command line. Start using the format
          `MANUAL:PART:CHAPTER:SECTION'.  Some documentation backends
          might not provide support for PART structuring. So the regular
          expression defining the documenation entry was also updated to
          support the format `MANUAL::CHAPTER:SECTION' for those cases
          where PART structuring isn't supported.
    
        - Add MANUAL_PART array variable to store part information
          retrived from documentation entry provided to `centos-art.sh'
          script command-line.
    
        - Add MANUAL_PART_NAME, MANUAL_PART_DIR variables to store part
          related information.
    
        - Redefine MANUAL_CHAPTER_DIR using MANUAL_PART_DIR and be sure no
          doble slash be present for those cases when MANUAL_PART_NAME is
          empty.
    
        - Move MANUAL_EXTENSION variable definition from help.sh to
          Texinfo/texinfo.sh (i.e., make it specific to documentation
          backends in order for it to be able of being redefined to
          something else but the documentation backend string itself.)
    
        - Add verification inside texinfo documentation backend to be sure
          no PART structuring be set through the documentation entry
          provided to `centos-art.sh' script command-line. There is no
          support for part structuring on texinfo-4.8.
    
        - Update comments.