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.
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'