children_kill_cb() is racey. One obvious problem is that
process_close_handles() is *queued* by on_process_exit(), so when
children_kill_cb() is invoked, the dead process might still be in the
`loop->children` list. If the OS already reclaimed the dead PID, Nvim
may try to SIGKILL it.
Avoid that by checking `proc->status`.
Vim doesn't have this problem because it doesn't attempt to kill
processes that ignored SIGTERM after a timeout.
closes#8269
8fa0b8051d
