Keyringrs

- Add combination keystore of keyutils and secret service (thanks to soywod).

- Add debug logging of internal operations (thanks to soywod).
- Revert iOS security-framework dependency to v2 (see 225).

- Allow use of both secret-service and keyutils.

- Add support for credential-store attributes other than those used by this crate. This allows the creation of credentials that are more compatible with 3rd-party clients, such as the OS-provided GUIs over credentials.
- Make the textual descriptions of entries consistently follow the form `userservice` (or `userservice:target` if a target was specified).

- Re-enable access to v1 credentials. The fixes of version 3.2 meant that legacy credentials with no target attribute couldn't be accessed.

- Improve secret-service handling of targets, so that searches on locked items distinguish items with different targets properly.