make the pruning at each name lookup, only in Curl_done().

my_getaddrinfo() has been called (and wait has been set to TRUE), we check
if the name already is resolved and if so don't return wait status to the
parent. This can happen with IP-only names.

build libcurl to use ares for asynch name resolves.

the define accordingly in the hostip.h header to work nicely all over.

for freeing single dns cache entries

counter so that the caller needs to decrease that counter when done with
the returned data.

If compiled with MALLOCDEBUG I've added some extra checking that the counter
is decreased before a handle is closed etc.

any name resolved data in any curl handle struct. That way, we won't mind
if the cache entries are pruned for the next time we need them. We'll just
resolve them again instead.

This changes the Curl_resolv() proto. It modifies the SessionHandle struct
but perhaps most importantly, it'll make the internals somewhat dependent
on the DNS cache not being disabled as that will cripple operations somewhat.
Especially for persistant connections.

    - Use a global dns cache (via setting the tentatively named,
    CURLOPT_DNS_USE_GLOBAL_CACHE option to true)
    - Use a per-handle dns cache, by default
    - Use a pooled dns cache when in the "multi" interface

be posted in a minute to the libcurl list.