Add two implementations of getaddrinfo: one blocking and one nonblocking.

Add two implementations of getaddrinfo: one blocking and one nonblocking.

The entry points are evutil_getaddrinfo and evdns_getaddrinfo respectively.
There are fairly extensive unit tests.

I believe this code conforms to RFC3493 pretty closely, but there are
probably more issues.  It should get tested on more platforms.

This code means we can dump the well-intentioned but weirdly-implemented
bufferevent_evdns and evutil_resolve code.

svn:r1537