fetcher: Return NOT_FOUND when the HTTP code is 410 or 404

This will be used by the pull code to download optional data.
trivial-httpd: Handle -p - as meaning write-port-to-stdout

This is convenient to use from tests.
trivial-httpd: Close stdout & stdin so $() can capture output when daemonized

Will be used by tests.
trivial-httpd: Handle the autoexit case when the docroot is a symlink