scd: Support reading the Yubikey 4 firmware version.

* scd/app.c (app_new_register): Detect yk4 version numbers.

Having the version of the yubikey is important to select which other
methods can be used with a Yubikey. Note that we do not detect the
formfactor of a Yubikey 4 and instead use 0 for our serial number
prefix. This does not affect app-openpgp becuase there we use the app
specific serial number.

• Signed-off-by: Werner Koch <wk@gnupg.org>