icmpv6: Pass prev to NDP options

Options are created via helper _parse_ndp_options() which wasn't
setting the prev packet, but should now.
libopenflow: Improve handling of multipart stats bodies

Some stats bodies are actually a list of entries and not a single entry.
We now handle those better.
datapaths: Fix parsing of stats requests

This code hadn't been updated since improvements to libopenflow and was
trying to unpack stats request bodies which were already unpacked.

Also cleans up some comments and a bad exception re-raising.
datapaths: Fix port stats reply for OFPP_NONE

The "all ports" stats were straight up wrong, apparently due to a
mis-reading of the spec.  Fixed.