telnet: fix strage blank spaces at beginning of telnet lines

telnet: fix strage blank spaces at beginning of telnet lines

Sometimes we saw two strange blank spaces at the beginning
of the telnet lines.

progress
  ogress
>

This patch fixes this problem:

progress
progress
>

The code changes are *reasonably* clean, but perhaps it could be
made a bit more elegant, but I didn't want to change things after
I finished diagnosis/testing & submitting the patch.

The problem was that logging can send the text and the newline
separately in two different requests and the telnet code would
incorrectly remove the prompt from the end of a line.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>