Fix missing AC_PROG_SED on older Autoconfs

For pre-2.59b Autoconfs, AC_PROG_SED is not available [1]; on such
systems, avoid calling AC_PROG_SED, while providing a sensible SED.

This aids backporting to Autoconf 2.59.

[1] http://lists.gnu.org/archive/html/autotools-announce/2004-08/msg00002.html
Backport libevent to vanilla Autoconf 2.59 (as used in RHEL5)

This is a backport of ad03952.
event_base_active_by_fd
POST supported, args supported
Merge branch 'master' of https://github.com/libevent/libevent into https
https-client: code cleanup
event_base_update_cache_time should be a no-op if the loop isn't running
evdns: New flag to make evdns not prevent the event loop from exiting

Here is the brief description of problem:
When you are use evdns to resolve domains to IP adresses (see
./sample/dns-example) you loop never returns from event_base_dispatch(),
and because of this the program will never terminated.

Because existing programs may be depending on the old behavior, we
only apply the fix when evdns_base_new() is created with a new flag -
EVDNS_BASE_DISABLE_WHEN_INACTIVE.

 (Commit message edited by Nick while squashing the branch.)
Merge branch '21_evdns_disable_when_inactive'
Merge remote-tracking branch 'origin/pr/36' into patches-2.0
Merge remote-tracking branch 'origin/patches-2.0'

Conflicts:
	configure.ac
Fix a warning introduced in 0c6ec5d8
Fix another warning introduced in 0c6ec5d8
Add new error_cb for actual reporting of HTTP request errors.

It is useful to know why you callback called with NULL (i.e. it failed),
for example if you set max_body with evhttp_connection_set_max_body_size()
you must know that it failed because of body was longer than this size.

 (Commit message tweaked by Nick)
Add test for EVREQ_HTTP_REQUEST_CANCEL into http_cancel_test()
Drop extra header http_struct.h from regress_http.c
Merge branch '21_http_error_cb_squashed'
Make all tests pass under EVENT_DEBUG_MODE=1

The fix was easy: the tests that were failing were those tests that
also enabled debug mode themselves. So, let them know when
EVENT_DEBUG_MODE is on, so they won't do that.
Implement event_finalize() and related functions to avoid certain deadlocks
Add some verbose notes to bufferevent unit tests