Make LineEditor::get_line() responsible for printing the prompt. That way
we can re-prompt after clearing the screen on ^L.
This makes the Serenity Terminal feel a little bit more like home :^)
Also run it across the whole tree to get everything using the One True Style.
We don't yet run this in an automated fashion as it's a little slow, but
there is a snippet to do so in makeall.sh.
