FIDO U2F: Register 'exclude_credentials'

[AlfioEmanueleFresta]

CTAP1 does not support 'exclude credentials' in its register operation (like CTAP2). However, this functionality can be be implemented by exploiting the key enumeration functionality of the protocol. Before the register operation, for each key in the exclude list, the client sends a 'check only' signature request. If any of the keys exist, the register operation is aborted, with a compatible error.

This feature is needed to support CTAP2 request downgrading.