Fix enumerate_bootstrap_servers:

- in py27 socket.getaddrinfo() has no kwargs
 - in py3 socket.inet_aton() raises OSError if no IPv6

src/helper_startup.py

@@ -393,15 +393,17 @@ def start_proxyconfig():
 
 
 def enumerate_bootstrap_servers():
-    """Generator of (IP, port) pairs resolved from dns seeds"""
+    """Generator of (IP, port) pairs resolved from DNS seeds"""
     for port in [8080, 8444]:
         for item in socket.getaddrinfo(
-            'bootstrap%s.bitmessage.org' % port, port, type=socket.SOCK_STREAM
+            # , type=socket.SOCK_STREAM
+            'bootstrap%s.bitmessage.org' % port, port
         ):
             try:
                 peer = item[4]
                 socket.inet_aton(peer[0])
-            except (TypeError, socket.error):
+            except (OSError, TypeError, socket.error):
                 continue
             else:
-                yield peer[:2]
+                if item[1] == socket.SOCK_STREAM:  # only once each pair
+                    yield peer[:2]