Cleanup of DC enumeration in get_dcs()

Cleanup of DC enumeration in get_dcs()

This is a fix for a few small inefficiencies/bugs in the get_dcs() path.

* because the third add_one_dc_unique() loop was outside the ADS check all DCs
  returned from the non-sitename lookup were being tacked onto the dc_name_ip
  list twice.
* add_one_dc_unique() now checks if the given IP address already exists before
  adding it to the list, making the returned list actually unique
* added more thorough doxygen comment headers