Do not sleep the full kill_delay when using send_interrupt and the current application shuts down by itself

[istyf]

Currently when send_interrupt is configured to true (for linux and unix only), the killCmd function sleeps for kill_delay before following up with a kill command as well.

If the target process honours the initial interrupt, cleans up and exits by itself, there is no need to sleep for a delay to try to kill a process that is no longer there. Air should instead wake up and continue as soon as the process has closed down, and only go on to send a kill to processes that ignore the interrupt or take too long to shut down.

[hungtcs]

+1 Very useful boost, current behavior wastes too much time.