CURLOPT_XFERINFOFUNCTION: introducing a new progress callback

CURLOPT_XFERINFOFUNCTION is now the preferred progress callback function
and CURLOPT_PROGRESSFUNCTION is considered deprecated.

This new callback uses pure 'curl_off_t' arguments to pass on full
resolution sizes. It otherwise retains the same characteristics: the
same call rate, the same meanings for the arguments and the return code
is used the same way.

The progressfunc.c example is updated to show how to use the new
callback for newer libcurls while supporting the older one if built with
an older libcurl or even built with a newer libcurl while running with
an older.

lib/urldata.h

@@ -1428,7 +1428,8 @@ struct UserDefined {
   curl_read_callback fread_func;     /* function that reads the input */
   int is_fread_set; /* boolean, has read callback been set to non-NULL? */
   int is_fwrite_set; /* boolean, has write callback been set to non-NULL? */
-  curl_progress_callback fprogress;  /* function for progress information */
+  curl_progress_callback fprogress; /* OLD and deprecated progress callback  */
+  curl_xferinfo_callback fxferinfo; /* progress callback */
   curl_debug_callback fdebug;      /* function that write informational data */
   curl_ioctl_callback ioctl_func;  /* function for I/O control */
   curl_sockopt_callback fsockopt;  /* function for setting socket options */