Use parse_filename method to parse filenames throughout. Use get_root_dir to

Use parse_filename method to parse filenames throughout.  Use get_root_dir to
retrieve the current root directory throughout.  Set const in argument list,
where appropriate, throughout.
* choose.cc (set_existence): Detect case of uninstalled package with no
version.
(parse_filename): New function.
* filemanip.h (fileparse): New structure.
* mount.cc (read_mounts): Remember where we found the root mount.
(set_root_dir): New function.
(get_root_dir): Ditto.
* mount.h: Declare new functions.
* site.cc (get_root_dir_now): Renamed from get_root_dir.
(save_site_url): Use new function name.
(get_initial_list_idx): Ditto.