Further modification of previous "Block counter field not required as part of AES-CTR IV" patch.

This patch makes the library accept two forms of IV in AES-CTR mode:
- 12 byte IV (Nonce + ESP IV) - new, ipsec use case, library sets the block counter itself
- 16 byte IV (Nonce + ESP IV + Block counter) - old, block counter is set by the application

IV length is passed to the alogrithmic code and IV is initialized accordingly based on the length.