Add ability to specific accessibility on a per-item basis

[bvirlet]

We may want to change the accessibility of a given keychain item. However, the only way to do this currently with SAMKeychain is to use the class method [SAMKeychain accessibilityType]. This has two drawbacks:

• It sets the accessibility type for all the subsequent password saves
• If you want to change the accessibility type for a single save, you can rely on setting the global accessibility type because it wouldn't be thread-safe.

This PR adds an accessibility field on the SAMKeychainQuery. It doesn't otherwise change the API and stays backward compatible.

[bvirlet]

Hi everyone. Could this be merged since it has been approved?