Better handle interrupting TAP tests
Set up a signal handler for INT/TERM so that we run our END block if we
get them. In END, if the exit status indicates a problem, call
_update_pid(-1) to improve chances of the stop working in case start()
hasn't returned yet.
Also, change END's teardown_node() so that it passes fail_ok=>1, so that
if a node fails to stop, we still stop the other nodes in the same test.
Per complaint from Andres Freund.
This doesn't seem important enough to backpatch, at least for now.
Discussion: https://postgr.es/m/
20220930040734.mbted42oiynhn2t6@awork3.anarazel.de