- check cmd: run `sshd -t' to check sshd_config file

- reload precmd: run check cmd before reloading so we don't nuke sshd
  if there's an error in the sshd_config file

(It is still possible to effectively nuke sshd by changing the
configuration tosomething that won't work on your network, but at
least we avoid making sshd just exit on reload when you make a typo
in a config option.)

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