Extends code so that later API may support LUKS2 device
activation via token with specified keyslot.
Also allows testing if specific token is able to unlock specific
keyslot.
First, there was a bug where passphrase based
keyslot contextets did not cache volume keys
properly and caused leaks.
Second, it causes problems when keyslot context
is used twice with different keyslot id, e.g.:
CRYPT_ANY_SLOT vs specific id, unbound key vs
volume key, etc.
The crypt_keyslot_add_by_keyslot_context & associated
helper functions allow more options when adding new
keyslot. For example there was no simple way of
adding new LUKS2 keyslot when the only active keyslot
could be unlocked by passphrase (KEK) provided by LUKS2 token
(plugin). Now all available options for unlocking keyslots
may also be used when creating new keyslot and it combine
as called needs.
The available methods (keyslot contexts) are:
passphrase, keyfile, key (binary representation) and LUSK2 token.
