configurator: reloading with unset values restores default

If a configuration directive is set at startup and later
unset, it correctly restores the original default value
as if it had never been set in the first place.

This applies to the majority of the configuration values with
a few exceptions:

* This only applies to stderr_path and stdout_path when
  daemonized (the usual case, they'll be redirected to
  "/dev/null").  When NOT daemonized, we cannot easily redirect
  back to the original stdout/stderr destinations.

* Unsetting working_directory does not restore the
  original working directory where Unicorn was started.
  As far as we can tell unsetting this after setting it is
  rarely desirable and greatly increases the probability of
  user error.
(cherry picked from commit 51b2b90284000aee8d79b37a5406173c45ae212d)

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