Fix XTS, and name things a bit better...

Though confusing, GCM using ICM_BLOCK_LEN, but ICM does not is
correct... GCM is built on ICM, but uses a function other than
swcr_encdec... swcr_encdec cannot handle partial blocks which is
why it must still use AES_BLOCK_LEN and is why XTS was broken by the
commit...

Thanks to the tests for helping sure I didn't break GCM w/ an earlier
patch...

I did run the tests w/o this patch, and need to figure out why they
did not fail, clearly more tests are needed...

Prodded by: peter