Wait for "cred" and "conf" files in clients

[philip-davis]

Many (most?) job scripts out in the wild wait to launch clients until the necessary files have been created by the server. I think this functionality should be folded into the clients so that users don't have to know how this works. We could insert a check for existence and size > 0, perhaps with a short delay (or lock-checking?) to ensure the server has finished writing the file. We can have an informative time-out message as well, to make clear what went wrong.