file: Allow a file descriptor to be passed to the plugin

This allows you to pass in a file descriptor to the plugin, eg:

  $ exec 6<>/var/tmp/fedora-36.img
  $ ./nbdkit file fd=6 --run 'nbdinfo $uri'

Note this was previously possible on most platforms using /dev/fd/<N>,
but not all platforms that have file descriptors support this and it
seems better to make both the feature and its use explicit.

Thanks: Eric Blake