Fixed the leak responsible for the accumulation of unreferenced scalars

[creaktive]

The leak occurs due to creation of empty base object inside the XS code. This fix also happens to solve the "Attempt to free unreferenced scalar: SV 0xdeadbeef during global destruction." issue.
Modifications:

1. Reimplemented t/old-13slowleak.t as t/96-leak.t, using Devel::Leak. It now runs 10_000 iterations instead of 200. However, it's still fast, as the downloaded URL is "file:///$Bin/$Script" (by default). This was required to spot the problem.
2. Updated Makefile.PL so pkg-config is now able to detect libcurl in non-standard locations (like $HOME). This was required for me to deploy test cases on multiple perlbrew-based installations.
3. Updated inc/symbols-in-versions from libcurl/7.28.0 and created a .gitignore file. Not actually required to fix the leak, but hell, why not? :)
4. The fix itself: in Curl.xs, HASHREF_BY_DEFAULT definition was updated, reverting the order of newRV_noinc() / sv_2mortal().

The dismissal of *"Attempt to free unreferenced scalar..." warning can be verified via Net::Curl::Simple test suite; while using the original Net::Curl, it repeatedly produces the aforementioned message:

...
t/42-coro-anyevent.t ..... 1/20 Please rebuild libcurl with AsynchDNS to avoid blocking DNS requests
# loaded implementation: Net/Curl/Simple/Async/AnyEvent.pm
Attempt to free unreferenced scalar: SV 0x7fa88b964378 during global destruction.
t/42-coro-anyevent.t ..... ok
...

I also "smoked" the patched Net::Curl on a reasonably large crawler system (which processes over 1 million HTML-pages every 24 hours) no unexpected behavior was detected.

[dolmen]

Reverse link to rt.cpan.org: RT#81053