code rearrange to fit the future better.

(http://curl.haxx.se/bug/view.cgi?id=1431750) helped me identify and fix two
different but related bugs:

1) Removing an easy handle from a multi handle before the transfer is done
   could leave a connection in the connection cache for that handle that is
   in a state that isn't suitable for re-use. A subsequent re-use could then
   read from a NULL pointer and segfault.

2) When an easy handle was removed from the multi handle, there could be an
   outstanding c-ares DNS name resolve request. When the response arrived,
   it caused havoc since the connection struct it "belonged" to could've
   been freed already.

Now Curl_done() is called when an easy handle is removed from a multi handle
pre-maturely (that is, before the transfer was complteted). Curl_done() also
makes sure to cancel all (if any) outstanding c-ares requests.

Fix compiler warning and compatibility issue with the type of the parameter used in getnameinfo() to receive the length of the sockaddr struct.

from url.c: Curl_disconnect().

The tag 'before_ftp_statemachine' was set just before this commit in case
of future need.

CURLE_OK or CURLE_OUT_OF_MEMORY.
Add typecast in hostares.c.

of typecasts all over

linked list for name resolved data, even on hosts/systems with only IPv4
stacks as this simplifies a lot of code.

a host name that resolves to multiple IP addresses.

make the code easier to read

warnings

usage.

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