don't round-robin, that goes against linux conventions

Changes nameserver_pick to treat the nameservers as a priority list, instead of always using round-robin to pick the next anemserver to use
Here you start from `base->server_head`, and here https://github.com/libevent/libevent/pull/184/files?diff=split#diff-10cf121c57f11a2e1ceadf7cef0c935aR1341 you return not the first, I think that in that case the first must be returned too.