MINOR: cli: Use a full prompt command for bidir connections with workers

MINOR: cli: Use a full prompt command for bidir connections with workers

When a bidirection connection with no command is establisehd with a worker
(so "@@<pid>" alone), a "prompt" command is automatically added to display
the worker's prompt and enter in interactive mode in the worker context.
However, till now, an unfinished command line is sent, with a semicolon
instead of a newline at the end. It is not exactly a bug because this
works. But it is not really expected and could be a problem for future
changes.

So now, a full command line is sent: the "prompt" command finished by a
newline character.