changes to cam_xpt to support sequential scanning (!parallel)

[Matt Jacob]

One solution that Scott suggested to some of the negotiation issues for
the MPT was to not do parallel problem, i.e., all targets in parallel.

The patch at http://people.freebsd.org/~mjacob/CAM_PATCHES_FOR_SEQSCAN

does just that and I've had quite good success with it so far. I add
a new flat, PIM_SEQSCAN, which cases xpt_scan_bus to *not* scan all
targets in parallel but rather to scan them sequentially.

Comments?

The code also does some cleanups under failure conditions.

After a bit more testing I'd like to put this in if there are no
objections so I can get the mpt U320 stuff off my back for a while.