MFprojects/camlock:
r249017:
Some cosmetic things:

• Unify device to target insertion inside xpt_alloc_device() instead of

duplicating it three times.

• Remove extra checks for empty lists of devices and targets on release

since zero refcount check also implies it.

• Reformat code to reduce indentation.

r249103:

• Add lock assertions to every point where reference counters are modified.
• When reference counters are reaching zero, add assertions that there are

no children items left.

• Add a bit more locking to the xptpdperiphtraverse().