major rework of FIDO token selection logic

When PINs are in use and multiple FIDO tokens are attached to a host, we
cannot just blast requests at all attached tokens with the PIN specified
as this will cause the per-token PIN failure counter to increment. If
this retry counter hits the token's limit (usually 3 attempts), then the
token will lock itself and render all (web and SSH) of its keys invalid.
We don't want this.

So this reworks the key selection logic for the specific case of
multiple keys being attached. When multiple keys are attached and the
operation requires a PIN, then the user must touch the key that they
wish to use first in order to identify it.

This may require multiple touches, but only if there are multiple keys
attached AND (usually) the operation requires a PIN. The usual case of a
single key attached should be unaffected.

Work by Pedro Martelletto; ok myself and markus@

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