Fix bug in busdma: if segs is a preexisting buffer, we memcpy it
into the DMA map. The length of the buffer had not yet been
initialized, however, so this would copy gibberish unless it
happened to be right by chance. This bug mostly only affected
systems with IOMMUs.

Approved by:	re (gjb)
MFC after:	3 days
Add a loader tunable to use only device tree-provided PCI devices. This is
needed on some more fragile systems to avoid machine checks when blindly
probing the PCI bus. Also reduce ofw_pcibus's priority slightly so that it
can be overridden.

Approved by:	re (gjb)