SI-4563 friendlier behavior for Ctrl+D in the REPL

Closing the REPL with Ctrl+D does not issue a newline, so the user's
prompt displays on the same line as the `scala>` prompt. This is bad.
We now force a newline before closing the interpreter, and display
`:quit` while we're at it so that people know how to exit the REPL
(since `exit` doesn't exist anymore).

The tricky part was to only add a newline when the console is
interrupted, and *not* when it is closed by a command (like `:quit`),
since commands are processed after their text (including newline) has
been sent to the console.