cryptosoft: Fix support for variable tag lengths in AES-CCM.

The tag length is included as one of the values in the flags byte of
block 0 passed to CBC_MAC, so merely copying the first N bytes is
insufficient.

To avoid adding more sideband data to the CBC MAC software context,
pull the generation of block 0, the AAD length, and AAD padding out of
cbc_mac.c and into cryptosoft.c. This matches how GCM/GMAC are
handled where the length block is constructed in cryptosoft.c and
passed as an input to the Update callback. As a result, the CBC MAC
Update() routine is now much simpler and simply performs the
XOR-and-encrypt step on each input block.

While here, avoid a copy to the staging block in the Update routine
when one or more full blocks are passed as input to the Update
callback.

Reviewed by: sef
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32120