When failing, it should not tamper at all with the data it was supposed to
add to the cache.

out of memory (thanks to 'runtests.pl -t')

returns NULL

NOT free the existing hash entry when we try to add a new one that matches
an existing entry. We now instead free the new one, and make the parent
function use the old entry's struct instead.

more in the same style as other curl source code. The only actual code change
is an added check after a malloc() call.

2. don't leave allocated memory behind when returning error

removed emacs local-variables stuff

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.

rule: "Curl_" prefix for library-wide private symbols, "curl_" is for
exported symbols.

This can be optimized a tidbit, but this is a start.

Reminder for the future: when we're using malloc() we MUST include <stdlib.h>
as otherwise 64bit archs go bananas.

Bug report #517687

on the left side of comparisons...

    - 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