Please support running in the background after opening listening port

[joshtriplett]

pebble and pebble-challtestserv run in the foreground, and have to be explicitly placed in the background. Please consider adding an option to open a listening port and then go into the background, which would allow a race-free way to start the server and then start making requests to the server, for CI purposes.

[cpach]

Perhaps one solution could be to wrap pebble in a shell script that sends the process to the background?