Merge of r4915, r4916, r4917: upstream minor fixes.

Merge of r4915, r4916, r4917: upstream minor fixes.

*) Upstream: honor the "down" flag for a single server.

   If an upstream block was defined with the only server marked as
   "down", e.g.

       upstream u {
           server 127.0.0.1:8080 down;
       }

   an attempt was made to contact the server despite the "down" flag.
   It is believed that immediate 502 response is better in such a
   case, and it's also consistent with what is currently done in case
   of multiple servers all marked as "down".

*) Upstream: better detection of connect() failures with kqueue.

   Pending EOF might be reported on both read and write events, whichever
   comes first, so check both of them.

   Patch by Yichun Zhang (agentzh), slightly modified.