Update `help' functionality:
    
        - Move some variable definitions from `Help/Texinfo/texinfo.sh' to
          `Help/help.sh'.
    
        - The MANUAL_BACKEND information is no longer retrived from
          `--backend' option. Instead, the `manual_backend' option inside
          manual's configuration file is used. When the documentation
          manual is created for first time, a selection list is prompted
          with all documentation backends supported in order for the user
          to select one of them. 
    
        - Because MANUAL_BACKEND is now retrived from manual's
          configuration file, MANUAL_BACKEND variable definition was moved
          into the documentation entry interpretation loop; which, in
          turn, provokes that all dependent definitions be moved into the
          loop too.
    
        - The `help_updateOutputFiles' invocation was removed from
          `Help/help.sh' those places inside `Help/Texinfo/texinfo.sh'
          where it is needed. This invocation movement is required in
          order to control which backend-specific tasks does perform
          `help_updateOutputFiles' and which one doesn't.
    
        - Don't terminate script execution for functions like
          `texinfo_searchNodes', `texinfo_searchIndex' and
          `texinfo_updateOutputFiles' inside `Texinfo/texinfo.sh' file.
          Such action would prevent us from using those functionalities
          for more than one documentation entry passed to `centos-art.sh'
          script command-line.
    
        - Update comments.