Merge branch 'bufev-cancellations-v5'

This patch set fixes bufferevent via http request cancellations (connect() and
dns-request), it survives tests and cancel.. with --no-fork, so this must be ok
(though I have one patch for dns layer pending).
But I'm not sure about cancel.. unit tests on win32, will fix disable them
later if they will differs (plus maybe we must make them skip-by-default?).

Fixes: #333
* bufev-cancellations-v5:
  http: set fd to -1 unconditioally, to avoid leaking of DNS requests
  test/http: cover NS timed out during request cancellations separatelly
  http: avoid leaking of fd in evhttp_connection_free()
  http: get fd from be layer during connection reset
  be_sock: cancel in-progress dns requests
  evdns: export cancel via callbacks in util (like async lib core/extra issues)
  test/http: request cancellation with resolving/{conn,write}-timeouts in progress