client: exit if no jobs were sent and client is idle

client: exit if no jobs were sent and client is idle

If we invoke the client without arguments, it gets the probe
response and then waits for further response from the server.
But if no command line or job files were sent, the server
is idle. In that case, just quit.

Signed-off-by: Jens Axboe <axboe@kernel.dk>