Allow to create new files (but changing them is not allowed yet)

- since mediafire cannot deal with empty files, we put new files into a
   temporary location and upload them once they get closed
 - add create and write functions to fuse
 - pass a file handle to mfconn_api_upload_simple instead of a path
 - allow calc_sha256 to also compute the file size
 - error out when the key returned by upload/simple is empty
 - make valgrind.supp more lenient

fuse/hashtbl.h

@@ -73,4 +73,6 @@ bool            folder_tree_path_is_file(folder_tree * tree, mfconn * conn,
 int             folder_tree_open_file(folder_tree * tree, mfconn * conn,
                                       const char *path);
 
+int             folder_tree_tmp_open(folder_tree * tree);
+
 #endif