progress callback: can be called more than once per sec

Function pointer that should match the \fIcurl_progress_callback\fP prototype

found in \fI<curl/curl.h>\fP. This function gets called by libcurl instead of

its internal equivalent with a frequent interval during operation (roughly

once per second) no matter if data is being transfered or not.  Unknown/unused

argument values passed to the callback will be set to zero (like if you only

download data, the upload size will remain 0). Returning a non-zero value from

this callback will cause libcurl to abort the transfer and return

\fICURLE_ABORTED_BY_CALLBACK\fP.

once per second or sooner) no matter if data is being transfered or not.

Unknown/unused argument values passed to the callback will be set to zero

(like if you only download data, the upload size will remain 0). Returning a

non-zero value from this callback will cause libcurl to abort the transfer and

return \fICURLE_ABORTED_BY_CALLBACK\fP.

If you transfer data with the multi interface, this function will not be

called during periods of idleness unless you call the appropriate libcurl