Allow overwriting existing keys on YubiKey without a full reset

[ninegua]

If a YubiKey already has OpenPGP keys installed, gpg will actually prompt something different in an interactive session, which is not properly handled in the expect script in yubicopy.sh. This has to be fixed to improve script robustness, and offer support without a full reset (i.e. existing PINs and other settings can be kept).