TODO: have form functions use CURL handle argument

 15.6 remove CURLOPT_DNS_USE_GLOBAL_CACHE

 15.7 remove progress meter from libcurl

 15.8 remove 'curl_httppost' from public

 15.9 have form functions use CURL handle argument

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 Changing them to return a private handle will benefit the implementation and

 allow us much greater freedoms while still maintining a solid API and ABI.

15.9 have form functions use CURL handle argument

 curl_formadd() and curl_formget() both currently have no CURL handle

 argument, but both can use a callback that is set in the easy handle, and

 thus curl_formget() with callback cannot function without first having

 curl_easy_perform() (or similar) called - which is hard to grasp and a design

 mistake.