Starting in 4.0 the keychain will be using content protection

We will define 4 new classes for the keychain:

Ak = non_exportable, uid_protection|pin_protection
Ck = wrap_on_lock|unwrap_on_lock|non_exportable, uid_protection|pin_protection
Dk = non_exportable, uid_protection
Ek = wrap_on_lock|unwrap_on_lock|non_exportable, uid_protection|pin_protection

A new attribute will be added to all keychain items allowing a caller
to specify the content protection level.
Caller will have a choice or class Ak, Ck, Dk or Ek protection for the
items data.

Consider whether or not to allow SecItemUpdate to change the protection class.

Backup will no longer directly backup the keychain file, instead a new
SecKeychainMigrate() SPI will be added which takes a keybag and a password and
returns a keychain blob in which class A and C items are protected by the
provided keybags A and C keys, and the class E key protected items will not
migrate.

Restore will pass the keychain file from an old backup or keychain blob
returned by the new SPI and the backup keybag and password to securityd so
items can be migrated back into the system keychain db.  Class E items in the
backup set will only get restored if they are decryptable (restore is to same
device) otherwise they are discarded during the restore.


Implementors notes:
Migrate will take a db file needing to be upgraded or a keychain blob
needing to be migrated into the system security domain.