sfeed_update: add die() function for exit and cleanup, respect $TMPDIR

- Add a die() helper function to cleanup and exit.
  - NOTE that with an empty sfeedtmpdir the case rm -rf "" is fine.
- Respect $TMPDIR for creating temporary files like many UNIX tools do.
- Fix: when creating "${sfeedtmpdir}/ok" fails for some reason cleanup the
  whole temporary directory as well.
- Fix: when the feeds() function is not defined exit with status code 1 (this
  was incorrectly status code 0).
  Reproduce: sfeed_update /dev/null; echo $?

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