Log when a network connection succeeds.

Now I've got an enum for PlugLogType, it's easier to add things to it.
We were giving a blow-by-blow account of each connection attempt, and
when it failed, saying what went wrong before we moved on to the next
candidate address, but when one finally succeeded, we never logged
_that_. Now we do.

unix/uxnet.c

@@ -733,6 +733,10 @@ static int try_connect(NetSocket *sock)
          */
         sock->connected = true;
         sock->writable = true;
+
+        SockAddr thisaddr = sk_extractaddr_tmp(sock->addr, &sock->step);
+        plug_log(sock->plug, PLUGLOG_CONNECT_SUCCESS,
+                 &thisaddr, sock->port, NULL, 0);
     }
 
     uxsel_tell(sock);
@@ -1435,6 +1439,13 @@ static void net_select_result(int fd, int event)
                     }
                     if (!s->connected)
                         return;      /* another async attempt in progress */
+                } else {
+                    /*
+                     * The connection attempt succeeded.
+                     */
+                    SockAddr thisaddr = sk_extractaddr_tmp(s->addr, &s->step);
+                    plug_log(s->plug, PLUGLOG_CONNECT_SUCCESS,
+                             &thisaddr, s->port, NULL, 0);
                 }
             }