Telnet: add parameter to allow stripping the prompt

The "Read Until Prompt" and "Execute Command" keywords return the output
and including the prompt. This patch adds a new parameter "strip_prompt" to
these keywords which if set to true strips the prompt from the output.

Fixes #1874.
There's no need to use `True`, any non-empty string works fine too. Something like

    set `strip_prompt` to any true value, such as a non-empty string.

would be a little better.