when enrolling a resident key on a security token, check if a

when enrolling a resident key on a security token, check 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