Fixes#602.
I replaced the SIGKILL with a SIGINT, and then catch SIGINT with a
handler. This handler calls cancelSubs if necessary, and can later be
edited to perform other clean-up operations, too. I thought about, in
this same commit, changing the SIGTERM in
maybe_restart_mirroring_script to a SIGINT, but after tracing out the
code paths, I realized that isn't necessary. (The SIGTERM is
necessarily performed on a process that has not subscribed to any
zephyr classes, so cancelSubs is unnecessary. If we do think that we
may want to add additional clean-up operations in the future, though,
then it might be worth investigating changing this SIGTERM.)
(imported from commit 692b295be6cb40b0e4ec2ca0bc58c58056ed9bd9)
9ebfa84385