Let's make loading of keys a bit more automatic and define a common
place where key files can be placed. Specifically, whenever a volume of
name "foo" is attempted, search for a key file in
/etc/cryptsetup-keys.d/foo.key and /run/cryptsetup-keys.d/foo.key,
unless a key file is declared explicitly.
With this scheme we have a simple discovery in place that should make it
more straightfoward wher to place keys, and requires no explicit
configuration to be used.
This adds a new crypttab option for volumes "pkcs11-uri=" which takes a
PKCS#11 URI. When used the key stored in the line's key file is
decrypted with the private key the PKCS#11 URI indiciates.
This means any smartcard that can store private RSA keys is usable for
unlocking LUKS devices.
