Allow specification of a PIN from a file

[Firstyear]

Some applications don't have "user friendly ways to ask for a PIN during TPM2_PKCS11 load (for example, openvpn which requires patching to accept rfc pkcs11 uris) . It would be useful for TPM2_PKCS11 to allow specification of a PIN file from the environment. A variable like TPM2_PKCS11_PIN_FILE would be a good name, and this could even work with features like systemd credentials where the pin file can be set into this variable.