Migrate more SKIPIF -> EXTENSIONS (#7139)

This is a mix of more automated and manual migration. It should remove all applicable extension_loaded() checks outside of skipif.inc files.

Manually store CURLOPT_PRIVATE data

This is simpler than going through curl, because we can manage
the memory more easily (in particular, this removes the need for
to_free->str). Add

Manually store CURLOPT_PRIVATE data

This is simpler than going through curl, because we can manage
the memory more easily (in particular, this removes the need for
to_free->str). Additionally this allows the CURLOPT_PRIVATE data
to have any type, instead of being implicitly cast to string.

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