Better handle FIDO keys on tokens that provide user verification (UV)

Better handle FIDO keys on tokens that provide user verification (UV)
on the device itself, including biometric keys.

Query the token during key creation to determine whether it supports
on-token UV and, if so, clear the SSH_SK_USER_VERIFICATION_REQD flag
in the key so that ssh(1) doesn't automatically prompty for PIN later.

When making signatures with the key, query the token's capabilities
again and check whether the token is able (right now) to perform user-
verification without a PIN. If it is then the PIN prompt is bypassed
and user verification delegated to the token. If not (e.g. the token
is biometric capable, but no biometric are enrolled), then fall back
to user verification via the usual PIN prompt.

Work by Pedro Martelletto; ok myself and markus@

NB. cranks SSH_SK_VERSION_MAJOR