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.
Apparently there are some people using ECB.
This mode by design do not use any IV, unfortunately
kernel dmcrypt allows to specify them (but userspace crypto api don't).
Let support activation as it was in previous version.
Should fix issue#238.
