Make km_alloc(9) use the direct map for all "phys contig" mappings requested

Make km_alloc(9) use the direct map for all "phys contig" mappings requested
by the caller on architectures that implement them.  Make sure that we
physically align memory such that we meet any demands on virtual alignment
in this case.  This should reduce the overhead of mapping large pool pages
for pools that request dma'able memory.

ok deraadt@, dlg@