opennds: Release v6.0.0

opennds: Release v6.0.0

  * This version - for Openwrt after 19.07 - for compatibility with new MHD API
  * Set - minimum version of MHD to 0.9.71 for new MHD API [bluewavenet]
  * Set - use_outdated_mhd to 0 (disabled) as default [bluewavenet]
  * Add - Multifield PreAuth login script with css update [bluewavenet]
  * Add - Documentation and config option descriptions for configuring Walled Garden IP Sets

Signed-off-by: Rob White <rob@blue-wave.net>