Add pkcs11 command

[Antonio Iacono]

There are some good tools for pkcs11, like pkcs11-tool of the OpenSC
project, but often only need the list of key ids to perform signature
operations with the engine.

I would propose a new pkcs11 command which, for now, only makes the
list of ids and labels of keys present in a token.

I have already prepared a draft in this branch
https://github.com/opensignature/openssl/tree/add-pkcs11-command/apps

Thanks,
Antonio