if a credential with matching application and user ID strings already exists.
if so, prompt the user for confirmation before overwriting the credential.

patch from Pedro Martelletto via GHPR329

NB. cranks SSH_SK_VERSION_MAJOR, so any third-party FIDO middleware
implementations will need to adjust

OpenBSD-Commit-ID: e45e9f1bf2b2f32d9850669e7a8dbd64acc5fca4