Through a combination of insufficient variable initialization and
imprudent reuse of static buffers, the end-of-transfer statistics
displayed when stdout is not a tty always ended up as 0 B / 0 Bps.
Reorganize the code to use caller-provided buffers, tweak the ETA
display a bit, and reduce the visual differences between the tty and
non-tty end-of-transfer displays.

PR: 202424
Approved by: re (gjb@)