* major code cleanup (part 1)

* major code cleanup (part 1)

        - unused variables removed
        - unused functions removed
        - uninitialized variables checked
        - missing return statements added
        - missing function return types added
        - added parentheses around assignment and
          && in || when suggested by gcc

* src/mh.c
* src/folder.c
        replaced direct calls to folder->scan_tree
        with folder_scan_tree and restore folder
        prefs in this function. The folder should
        not have to deal with them.
        ( Should now work for all folder types )