First attempt at migrating to libevent2
- so far it's not working
Second attempt at migrating to libevent2
- still not working
- remember TODO in httpd.c chunk reply
Drop local copy of evhttp and rely on libevent's public headers only.

This currently does not work with any official libevent release, but
hopefully the local patches will get accepted and the next release of
libevent will contain the new funtions.