Support crypt segment in libdevmapper tree.

Support crypt segment in libdevmapper tree.

- it can support multiple segments, but note that
to work properly, correct IV (initialization vector)
offset parameter must be set properly.

Because most usage of IV start offset is when we join
several crypto segments together (so iv_offset is the segment
start offset), DM_CRYPT_IV_DEFAULT is defined to simplify
the process.

Function accepts the string in cipher agrument (already
including chainmode and iv type; chainmode and iv parameters are NULL
in this case) or user can provide split parameters which will
join into dm-crypt cipher specification "cipher-chainmode-iv".

All these parameters must be supplied in correct dm-crypt format.