Make built-in hostname validation of LibreSSL work #115

Ideally, 94924cf should not be necessary.

Currently, we are falling back to our internal (partial) validation when using LibreSSL. However, it should work just as well or even better with the built-in method of LibreSSL instead. We need to find out why some tests currently fail when removing this workaround. In particular, without this patch, the callback is apparently not called in some cases, which is very bad.

For more information, please also see SWI-Prolog/swipl-devel#249 (comment).

I am filing this as an issue to track its progress.