correct TCP error handling in dispatch and resolver

correct TCP error handling in dispatch and resolver

- certain TCP result codes, including ISC_R_EOF and
  ISC_R_CONNECTIONRESET, were being mapped to ISC_R_SHUTTINGDOWN
  before calling the response handler in tcp_recv_cancelall().
  the result codes should be passed through to the response handler
  without being changed.

- the response handlers, resquery_response() and req_response(), had
  code to return immediately if encountering ISC_R_EOF, but this is
  not the correct behavior; that should only happen in the case of
  ISC_R_CANCELED when it was the caller that canceled the operation

- ISC_R_CONNECTIONRESET was not being caught in rctx_dispfail().

- removed code in rctx_dispfail() to retry queries without EDNS
  when receiving ISC_R_EOF; this is now treated the same as any
  other connection failure.