Improve support for stream ciphers in the software encryption interface.

Add a 'native_blocksize' member to 'struct enc_xform' that ciphers can
use if they support a partial final block. This is particular useful
for stream ciphers, but can also apply to other ciphers. cryptosoft
will only pass in native blocks to the encrypt and decrypt hooks. For
the final partial block, 'struct enc_xform' now has new
encrypt_last/decrypt_last hooks which accept the length of the final
block. The multi_block methods are also retired.

Mark AES-ICM (AES-CTR) as a stream cipher. This has some interesting
effects on IPsec in that FreeBSD can now properly receive all packets
sent by Linux when using AES-CTR, but FreeBSD can no longer
interoperate with OpenBSD and older verisons of FreeBSD which assume
AES-CTR packets have a payload padded to a 16-byte boundary. Kornel
has offered to work on a patch to add a compatiblity sysctl to enforce
additional padding for AES-CTR in esp_output to permit compatibility
with OpenBSD and older versions of FreeBSD.

AES-XTS continues to use a block size of a single AES block length.
It is possible to adjust it to support partial final blocks by
implementing cipher text stealing via encrypt_last/decrypt_last hooks,
but I have not done so.

Reviewed by: cem (earlier version)
Tested by: Kornel Dulęba <mindal@semihalf.com> (AES-CTR with IPsec)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D24906