Track actual bytes read in read operations

Add functionality to determine the actual number of bytes read during
read operations, making it possible to detect when a read was truncated
because it went past the end of the file.

Changes:
- Modified file_tree_read() to calculate and return actual_bytes via
  new output parameter
- Updated metadata server to send actual_bytes in READ_SUCCESS messages
- Added bytes_read field to ToastyResult structure
- Modified client to parse, store, and report actual bytes read
- Updated toasty_read() to return the actual number of bytes read
  instead of always returning 0
- Fixed web server to use bytes_read field instead of non-existent
  count field

This allows clients to distinguish between:
- Reading zeros because the file is sparse (has holes)
- Reading past the end of the file (truncated read)

src/file_tree.h

@@ -56,7 +56,7 @@ int    file_tree_list          (FileTree *ft, string path, ListItem *items, int
 int    file_tree_create_entity (FileTree *ft, string path, bool is_dir, uint64_t chunk_size);
 int    file_tree_delete_entity (FileTree *ft, string path);
 int    file_tree_write         (FileTree *ft, string path, uint64_t off, uint64_t len, uint32_t num_chunks, uint32_t chunk_size, SHA256 *prev_hashes, SHA256 *hashes, SHA256 *removed_hashes, int *num_removed);
-int    file_tree_read          (FileTree *ft, string path, uint64_t off, uint64_t len, uint64_t *chunk_size, SHA256 *hashes, int max_hashes);
+int    file_tree_read          (FileTree *ft, string path, uint64_t off, uint64_t len, uint64_t *chunk_size, SHA256 *hashes, int max_hashes, uint64_t *actual_bytes);
 string file_tree_strerror      (int code);
 int    file_tree_serialize     (FileTree *ft, int (*flush_fn)(char*,int,void*), void *flush_data);
 int    file_tree_deserialize   (FileTree *ft, int (*read_fn)(char*,int,void*), void *read_data);