Make OF_nextprop() work correctly for FDT by using the libfdt
fdt_next_property_offset() API. The old code would sometimes (e.g. on
the device tree supplied by the RB800 boot loader) get confused and stop
partway through listing properties.

MFC after:	1 week
Do not panic if pmap_mincore() is called. This prevents crashing userland
binaries from bringing down the kernel.

MFC after:	3 days
An addendum: it is possible, though of questionable utility, for a node
to have no properties at all. Do the right thing in such cases.