Added automatic field de-/encryption.

- Added auto_crypt option to packets. If enabled, encrypted fields (such
  as User-Password or Tunnel-Password) in Auth Packets will be
  automatically de-/encrypted if accessed via the key from the radius
  dictionary.
- Fixed bug encrypting tunnel passwords: The ciphertext is now prefixed
  with the salt used for encryption.
Added auto_crypt functionality to Server.