opencrypto: Add mechanism to pass multiple crypto blocks to some ciphers

xforms that support processing of multiple blocks at a time (to support more
efficient modes, for example) can define the encrypt_ and decrypt_multi
interfaces. If these interfaces are not present, the generic cryptosoft
code falls back on the block-at-a-time encrypt/decrypt interfaces.

Stream ciphers may support arbitrarily sized inputs (equivalent to an input
block size of 1 byte) but may be more efficient if a larger block is passed.

Sponsored by: Dell EMC Isilon