CURLOPT_IPRESOLVE: preventing wrong IP version from being used

In some situations, it was possible that a transfer was setup to
use an specific IP version, but due do DNS caching or connection
reuse, it ended up using a different IP version from requested.

This commit changes the effect of CURLOPT_IPRESOLVE from simply
restricting address resolution to preventing the wrong connection
type being used, when choosing a connection from the pool, and
to restricting what addresses could be used when establishing
a new connection.

It is important that all addresses versions are resolved, even if
not used in that transfer in particular, because the result is
cached, and could be useful for a different transfer with a
different CURLOPT_IPRESOLVE setting.

Closes #6853

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