Improve r238673 to additionally allow for odd-aligned buffers as

Improve r238673 to additionally allow for odd-aligned buffers as
passed in by smartd of smartmontools.
While at it, hint the compiler that 32-bit PIO is the most likely
case (idea from Linux) and use bus_{read,write}_stream_2(9) instead
of bus_{read,write}_multi_stream_2(9) for single count reads/writes.

MFC after:	1 week