Add support to help and help_ to use -referencing_dir to locate info segments

[Eric Swenson]

Since the info search path already supports -referencing_dir and this is included in the default search path, it might be nice to add the following feature: if "help foo" is invoked, and foo is a program found in the search rules, but in a non-standard library, and if -referencing_dir is present in the info search paths, then look for a "foo.info" segment in the same directory where "foo" is found.

[Gary Dixon]

Here is a plan for implementing this enhancement.

Implement Ticket #292: change help_ to implement a default -referencing_dir
search mechanism, as follows.

 A) Change help_$init:
    IFF:
      - search_list_name parm identifies a search list w/ -referencing_dir 
        search rule; AND
      - search_list_ref_dir parm is empty string 
    THEN:
      - expanded list of search directories will include a path set to:
          -referencing_dir at that point in search directory array.

 B) Change help_:
    WHEN searching for INFO_NAME.info or INFO_NAME
          - not having dir part (no absolute/relative dir part appears
            in INFO_NAME); AND
          - removing any .info suffix yields a 1-component entry name ...

    IF   reaching a -referencing_dir rule in search directory array...

    THEN use linker search paths to look for an INFO_SEG_NAME executeable
         segment pathname ...

         IFF:
          - matching executeable segment is found; AND
          - its dirname is not a top-level directory (e.g., >sss, >t, >sl1, etc.)
            (that is, it's dirname contains two or more > chars) ...
         THEN:
          - use that dirname in place of -referencing_dir search rule: 

              IFF:
               - INFO_NAME.info exists in that pathname...
              THEN:
               - select that segment for display by help_.
              ELSE:
               - skip the -referencing_dir search rule when searching for 
                 given INFO_NAME.