- Add check for df path for dvd_freespace

- Use df to get space used on DVD.
- Change sense of flag indicating erase DVD or not before writing.
- Fix bpipe so that it never modifies the result pointer.
- Replace more dev_name by print_name().
- Rewrite edit_device_codes_dev() so it does not overwrite the
  supplied buffer.
- Update printing of labels (for bls) so that critical information
  is printed (Job name and timestamp).
- Cleanup old spool files when starting the SD.
- Modify vss.cpp to allow C: as a path name.  Otherwise VSS
  doesn't work when only a drive name is given.
- Modify vss.c to eliminate double / in filenames.
- Update doc -- particularly the restore chapter.