A daemon killed in the second between spawning `hermes update` and that
child taking the update lock left a marker whose only identity was the dead
script: every reader judged it DEAD and a second update could start over the
first. The child now runs asynchronously just long enough for its pid to be
written as line 4 (`delegate:<pid> ct:<ct>`, compare-and-swap on our own
lines 1-3), then is waited on; HUP/INT/QUIT that land meanwhile are handled
after it exits, exactly as with the former foreground child.
(cherry picked from commit a09d2fb7cac0d730b491560ea0f970e5d227421e)
2245190411