Fix client's command abort feature

Fix client's command abort feature

The server aborts the current command when it receives a special line
of input for a prompt.  To make the client send it, you type the INTR
character (normally ^C).  This sends the client the SIGINT signal.
Unfortunately, it never quite worked.

Because we use a special line of input to signal interrupt, the client
can do that only after a complete line of input.

What if SIGINT arrives in the middle of a line?  We split the line in
two then and there, by inserting a newline.  Nasty, but it's simple,
and happens rarely.

However, we inserted the newline always, even after a complete line.
In that case, we inserted an empty line of input before the interrupt.
If you hit INTR at a server prompt, the server received an empty line
of input for that prompt, and the interrupt only for the *next*
prompt.  Which may well be too late to abort the command you wanted to
abort.

Fix by inserting the newline only when needed.