Add a way to keep relaunching the payload process in case it exits

[probonopd]

Add a way to keep relaunching the payload process in case it exits with an exit code other than 0.

Use case:
launch --relaunch-on-exit Filer --desktop could be used in the startup script. If Filer crashes, it would get automatically restarted. (Reference: helloSystem/hello#172 (comment))

Note:
The time between two non-successful launches should double every time it is tried (to prevent from "respawning too fast" situations due to launch-crash-launch-crash loops).

• Try to relaunch immediately after the payload process has exited with an exit code other than 0
• In case the relaunch fails, wait for 2 seconds and retry
• In case the relaunch fails again, wait for 4 seconds and retry
• In case the relaunch fails again, wait for 8 seconds and retry
• and so on
• Once the relaunch succeeds and the payload application does not exit with an exit code other than 0 within a certain amount of time, reset the delay so that in case the application crashes again, the whole process starts again.

Is this a sensible behavior?