restore pick_a_port() behind --port, WEBDIFF_PORT
restore pick_a_port() behind --port, WEBDIFF_PORT

nice to allow users to not have:

1. any cfg set up and still get a port that works, and
2. not have to have 8080 free necessarily
daytonb/webdiff,danvk/webdiff,daytonb/webdiff,daytonb/webdiff,daytonb/webdiff,danvk/webdiff,danvk/webdiff,danvk/webdiff,danvk/webdiff