Terrasync: implement HTTP service lookup via DNS

Terrasync: implement HTTP service lookup via DNS

Let terrasync use the http repository if
  /sim/terrasync/http-server == "automatic"
  resolve DNS NAPTR RR for terrasync.flightgear.org with service "ws20"
  and flags "U". Pick one of the returned entries with lowest order
  and preference. If more than one entry  with the same order and
  preference is returned, pick a random entry of those returned.

or if
  /sim/terrasync/http-server != "automatic"
  explicitly use the value of that property as the http server

if
  /sim/terrasync/http-server is empty, fall back to the legacy
  SVN repository