flatcar-update: Don't end with SIGTERM

The cleanup of all subprocesses through "kill 0" also ends up sending a
SIGTERM to the script itself, which prevents ending with a successful
return code.
Keep track of spawned subprocesses (at least the top ones) and only kill
them.

bin/flatcar-update

@@ -239,8 +239,12 @@ tee -a /tmp/response > /dev/null <<-EOF
 	</updatecheck><event status="ok"></event></app></response>
 EOF
 
-trap "umount /usr/share/update_engine/update-payload-key.pub.pem 2> /dev/null || true; rm -f /tmp/response /tmp/payload-server ; kill 0" EXIT INT
+true > /tmp/payload-server-pids
+trap "umount /usr/share/update_engine/update-payload-key.pub.pem 2> /dev/null || true; rm -f /tmp/response /tmp/payload-server ; cat /tmp/payload-server-pids | xargs -r kill ; rm -f /tmp/payload-server-pids" EXIT INT
 ncat --keep-open -c "echo -en 'HTTP/1.1 200 OK\ncontent-type: text/xml\ncontent-length: $(stat --printf='%s\n' /tmp/response)\n\n'; cat /tmp/response" -l "$LISTEN_PORT_1" &
+CHILDPID="$!"
+echo "${CHILDPID}" >> /tmp/payload-server-pids
+
 
 # Helper script because inline quoting is insane
 tee /tmp/payload-server > /dev/null <<'EOF'
@@ -267,6 +271,8 @@ EOF
 
 chmod +x /tmp/payload-server
 socat TCP-LISTEN:"${LISTEN_PORT_2}",reuseaddr,fork SYSTEM:'/tmp/payload-server /var/tmp/flatcar-update/ application/gzip' &
+CHILDPID="$!"
+echo "${CHILDPID}" >> /tmp/payload-server-pids
 
 if [ "${FORCE_DEV_KEY}" = "1" ] || [ "${FORCE_FLATCAR_KEY}" = "1" ]; then
   rm -f /tmp/key