This fixes crypt_keyslot_add_by_key where we were unable to store
keyslot (unbound to segment) with different key_size.
The code used (new) volume key size implicitly which could be wrong
if new size was not compatible with cipher parameter for keyslot area.
For now, crypto API quietly used cipher witout IV if a cipher
algorithm wihtou IV specificaton was used (e.g. aes-xts).
This caused fail later during activation.
This patch allows only two specific backed use without specified IV
(ECB mode and NULL cipher).
Also check cipher string early during parsing of CLI options.
