1. Don't check elapsed time in children_kill_cb(), it's already implied
by the start-time of the timer itself.
2. Restart timer from children_kill_cb() for PTY jobs, to send SIGKILL
after SIGTERM. There is an edge case where SIGKILL might follow
SIGTERM too quickly, if jobstop() is called near the 2-second timer
window. But this edge case is not worth code complication.
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