crypt_drop_keyring_key function allow to drop all keys in keyring
assocatiated with passed volume key list.
crypt_drop_keyring_key_by_description is used to drop independent key.
A keyslot not bound to any segment can store any key for any purpose.
To easily check slot status, new enum value is introduced.
This status is valid only for LUKS2, so the functions are backward compatible
with LUKS1.
Move all keyring functions to one place and separate LUKS2 specific
code to generic handling.
Also fix possible mismatch if volume key is in keyring but it is not native
LUKS2 device (libarary cannot process such a device properly).
Originally the key description for VK was derived
from segment id. This could lead to ambiguity when
keyslot key is verified and loaded in kernel keyring
using activation functions with CRYPT_ACTIVATE_KEYRING_KEY
flag raised.