Implement DNS prefetch mechanism à la Google Chrome.

The point is to speed up page access by resolving host names early.

Host names (up to 42 per page) found in links' hrefs are queued for early resolution at a moderate pace (one every 200ms) using KIO's DNS cache.

When TTL interval of DNS cache is reached and the page still exists, the anticipated resolution is performed again.

This mechanism can be restricted to resolution of only bare SLDs and www sub-domain, or may be entirely disabled using either the 'DNSPrefetch' configuration option or KHTMLPart's setDNSPrefetch() override.

It is also automatically disabled when setOnlyLocalReferences() has been set on the part (aka. "mail reader mode").