Adds `curl_off_t curl_multi_get_offt(CURLM *multi_handle, CURLMinfo_offt
info)` to the multi interface with enums:
* CURLMINFO_XFERS_CURRENT: current number of transfers
* CURLMINFO_XFERS_RUNNING: number of running transfers
* CURLMINFO_XFERS_PENDING: number of pending transfers
* CURLMINFO_XFERS_DONE: number of finished transfers to read
* CURLMINFO_XFERS_ADDED: total number of transfers added, ever
Add documentation for functions and info enums.
Add use in the curl command line tool to replace two static
variables counting the same "from the outside".
refs #17870Closes#17992
1ad2009ad6