Support for FD-side file encryption.

- Added MIN() and MAX() macros to baconfig.h
- Added stream types for win32, file, and macos encrypted data (with and without gzip)
- Added FO_ENCRYPT file flag.
- Implement crypto_cipher_* encryption functions
- Add encryption logic to save_file()

Decryption support has not been committed (yet):
If you use this code to encrypt your data, you will be unable to decrypt it!