Use off_t more consistently for file offsets and file size

Use off_t more consistently for file offsets and file size

- Change the seek and tell methods to off_t in the first place,
  and use fseeko/ftello throughout.
- Change class members holding file sizes and offsets to off_t,
  use matching casts.
- Use strtoll rather than sscanf to read octal file length
  in tar archives.  It's faster and typesafe.
- Use off_t as argument types for progress bar functions.

Additionally:

- Reformat io_stream_memory.h, which was next to unreadable
  due to bad formatting.