When downloading resident keys from a FIDO token, pass back the

When downloading resident keys from a FIDO token, pass back the
user ID that was used when the key was created and append it to
the filename the key is written to (if it is not the default).

Avoids keys being clobbered if the user created multiple
resident keys with the same application string but different
user IDs.

feedback Pedro Martelletto; ok markus

NB. increments SSH_SK_VERSION_MAJOR