RTT console: add --line-mode for line-buffered input

[ccattuto]

The RTT viewer puts the terminal into raw mode (~ICANON & ~ECHO) and
forwards every keystroke to the target's down buffer as it is typed.
A target that parses line-based commands therefore sees each partially
typed line and acts on it, often resulting in errors.

Add an -l/--line-mode option that buffers the line on the host and
submits it only when Enter is pressed. Backspace/delete and ctrl-U edit
the pending line, and the line is terminated with whichever of CR or LF
the user actually typed.

The flag is off by default, so that targets driven by single keystrokes keep
working unchanged. When it is off, the bytes written to the down buffer and
the characters echoed locally are identical to before.