gnupg-pkcs11-scd: init at 0.9.2

This adds gnupg-pkcs11-scd, a smart card daemon for GnuPG that supports
PKCS#11 smartcards (such as the Yubikey PIV module).

You can use it by adding something like this to your
~/.gnupg/gpg-agent.conf:

    scdaemon-program /home/<user>/.nix-profile/bin/gnupg-pkcs11-scd

You will also need to install `opensc` and have a
~/.gnupg/gnupg-pkcs11-scd.conf with something like the following:

    providers opensc

    provider-opensc-library /home/philandstuff/.nix-profile/lib/pkcs11/opensc-pkcs11.so

Then `gpg` smartcard operations will access your PKCS#11-capable
smartcard.