v3.2.0: Improve secret-service handling of targets

As reported in #201 and now fixed in #204, the secret-service credential store implementation could not disambiguate credentials that were created with different targets unless the collections containing those targets had already been unlocked.

The fix for this issue introduces a compatibility break: secret-service credentials created by the keyring v1 implementation, which were readable using platform-independent code in keyring v2, are no longer readable without using platform-specific code in v3. The necessary platform-specific code is very simple, and it has been added to the CLI (both for documentation purposes and to allow the CLI to access v1 credentials). The comments in issue #204 explain why the decision was made to add this secret-service-specific compatibility break; these include the fact that keyring v2 was released 18 months ago so only those users who haven't changed their password in a very long time would still have a v1 credential in their secret-service store.

The README has been updated to address this compatibility break and to provide recommendations for keyring clients who want to update their code to make sure users with old credentials are not impacted.