Add a bufferevent function to resolve a name then connect to it.

Add a bufferevent function to resolve a name then connect to it.

This function, bufferevent_socket_connect_hostname() can either use
evdns to do the resolve, or use a new function (evutil_resolve) that
uses getaddrinfo or gethostbyname, like http.c does now.

This function is meant to eventually replace the hostname resolution mess in
http.c.

svn:r1496