dd: append spaces to shorter status=progress line

dd: append spaces to shorter status=progress line

Problem noted by Pádraig Brady in: http://bugs.gnu.org/22277#8
Also, make the output a bit more precise while we're at it.
* NEWS: Document this.
* src/dd.c (previous_time): Remove, replacing with ...
(next_time): New var.  All uses changed.
This avoids some rounding errors, and should be a bit faster.
(newline_pending): Remove, replacing with ...
(progress_len): New var.  All uses changed.
This lets us keep track of how many trailing spaces to append.
(print_xfer_stats): Get the time first thing, so that it's
closer to being correct.  Count the bytes output, and append
trailing spaces if needed.  Add remarks to translators about
translation lengths.