Fix buffer overflows in progress 'bar' code

* src/progress.c (progress_interactive_p): Sanitize input.
  (progress_update): Likewise.
  (bar_create): Use larger BUF_LEN.
  (bar_create): Remove superfluous memset.
  (bar_create): Fix filename layout.
  (bar_create): Remove filename scrolling code, it caused many buffer
  overflows later in bar_create.
  (bar_create): Support TB/s download speed.