Update libevent to 2.0.19.

Changes in version 2.0.19-stable (3 May 2012)

* Refactor event_persist_closure: raise and extract some common logic
* If time has jumped so we'd reschedule a periodic event in the past, schedule
  it for the future instead
* If a higher-priority event becomes active, don't continue running events
  of the current priority.
* Fixed potential double-readcb execution with openssl bufferevents.
* Cancel a probe request when the server is freed, and ignore cancelled probe
  callbacks
* Remove redundant DNS_ERR_CANCEL check, move comment
* When retransmitting a timed-out DNS request, pick a fresh nameserver.
* Backport: provide EVENT_LOG_* names, and deprecate _EVENT_LOG_*

Changes in version 2.0.18-stable (22 Mar 2012)

* Make uses of open() close-on-exec safe by introducing an internal
  evutil_open_closeonexec.
* Properly zero the kevent in kq_setup_kevent()
* Stop crashing in evdns when nameserver probes give a weird error

Changes in version 2.0.17-stable (10 Feb 2012)

* Be absolutely sure to clear pncalls before leaving event_signal_closure
* check for sysctl before we use it
* Remove bogus casts of socket to int before calling ev_callback
* Make evconnlistener work around bug in older Linux when getting nmapped
* Fix a list corruption bug when using event_reinit() with signals present
* Fix a fd leak in event_reinit()
* Do a memberwise comparison of threading function tables
* Use C-style comments in C source files (for compatibility with compilers
  such as xlc on AIX).
* Avoid crash when freeing event_iocp and using event_set_mem_functions
* In the kqueue backend, do not report EBADF as an EV_READ
* Fix behavior of evbuffer_peek(buf,-1,NULL,NULL,0)
* Loop on filtering SSL reads until we are blocked or exhausted.
* Force strict validation of HTTP version in response.
* evdns: fix a bug in circular-queue implementation
* Backport evhttp_connection_get_bufferevent to Libevent 2.0

Changes in version 2.0.16-stable (18 Nov 2011)

* More detailed message in case of libevent self-debugging failure.
* epoll: close fd on alloc fail at initialization
* Fix compile warning from saying event2/*.h inside a comment
* Warn when unable to construct base because of failing make_base_notifiable
* Don't try to make notifiable event_base when no threading fns are configured
* unit test for remove_buffer bug
* Fix an evbuffer crash in evbuffer_remove_buffer()
* Refactor amount-to-read calculations in buffervent_ssl consider_reading()
* Move SSL rate-limit enforcement into bytes_to_read()
* Avoid spinning on OpenSSL reads
* Empty DNS reply with OK status is another way to say NODATA.

Changes in version 2.0.15-stable (12 Oct 2011)

* DNS: add ttl for negative answers using RFC 2308 idea.
* Add DNS_ERR_NODATA error code to handle empty replies.
* Make evbuffer callbacks get the right n_added value after evbuffer_add
* Prefer mmap to sendfile unless a DRAINS_TO_FD flag is set. Allows add_file
  to work with SSL.
* When a signal callback is activated to run multiple times, allow
  event_base_loopbreak to work even before they all have run.


(fhajny)