ftplistparser: move out private data from public struct

The public 'curl_fileinfo' struct contained three fields that are for
internal purposes only. This change makes them unused in the public
struct.

The new private struct fields are also renamed to make this separation
more obvious internally.

Closes #10844

lib/fileinfo.h

@@ -30,6 +30,9 @@
 struct fileinfo {
   struct curl_fileinfo info;
   struct Curl_llist_element list;
+  char *mem;
+  size_t size;
+  size_t used;
 };
 
 struct fileinfo *Curl_fileinfo_alloc(void);