WebPost aSub: implement progress function

A progress function as per https://curl.haxx.se/libcurl/c/CURLOPT_PROGRESSFUNCTION.html should be implemented in the form post routine #4000 as a better way to enforce a timeout

Start with the form post routine referenced by issue #4000 and read the linked libcurl CURLOPT_PROGRESSFUNCTION documentation. Trace how the routine currently handles timeouts, then verify that the progress callback provides the intended timeout enforcement and that the existing form-post behavior remains intact.

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