Re-implemented the block-preserving encryption restore. In the process, fixed all...

   - Removed the "chunk_size" implementation
   - Renamed crypto_count to crypto_buf_len, and crypto_size to crypto_packet_len.
   - Removed unnecessary looping in extract_data()
   - Broke out sparse block handling into its own function
   - Laid some groundwork for adding sparse + encryption support

Testing:
   - Encryption regression tests now pass.
   - Tested sparse file handling changes without encryption enabled.

Known encryption-related bugs remaining:
   - Restoring resource forks is broken; need to set up a seperate decryption context.
   - The latest changes to digest handling with sparse files breaks file signatures.
   - Sparse files are not currently supported when encrypting.