Implemented IPv6 support for URLs specified using domain names.

This includes "debug_connection", upstreams, "proxy_pass", etc.
(ticket #92)

To preserve compatibility, "listen" specified with a domain name
selects the first IPv4 address, if available.  If not available,
the first IPv6 address will be used (ticket #186).
diff --git a/src/core/ngx_inet.h b/src/core/ngx_inet.h
index e30dcb5..6a5a368 100644
--- a/src/core/ngx_inet.h
+++ b/src/core/ngx_inet.h
@@ -87,7 +87,7 @@
     unsigned                  listen:1;
     unsigned                  uri_part:1;
     unsigned                  no_resolve:1;
-    unsigned                  one_addr:1;
+    unsigned                  one_addr:1;  /* compatibility */
 
     unsigned                  no_port:1;
     unsigned                  wildcard:1;