packet.llc: Only parse as Ethernet when OUI=0

Previously, we always tried to parse the rest of the frame as if it were
Ethernet, but this should only be done if the OUI is 0x000000.  For any
other OUI, LLC's .next is the raw remainder of the frame.
packet: Adjust conditions for not-packing unparsed packets

1705997e3 added logic to packet_base which stops unparsed packets from
attempting to re-pack, and just uses their .raw instead.  It assumed that
.next would be set to None in cases where this should happen, but this
isn't the case with ipv4, for example, which has it set to an empty bytes.

This patch now counts anything falsey for .next as okay for non-repacking.
It's possible that this isn't a great idea and that things like ipv4 should
explicitly set .next to None instead.
libopenflow: Use OFP_DL_TYPE_NOT_ETH_TYPE in from_packet()

I haven't noticed this in the spec, but it seems like at least for
matching, if the Ethernet frame doesn't have a valid ethertype, it should
always be treated as if it were OFP_DL_TYPE_NOT_ETH_TYPE.  (Based on
OFTest and maybe OVS.)
libopenflow: Match on SNAP-encapsulated ethertype

from_packet() now checks if the frame has a SNAP header and gets the
ethertype out of it if so.  Questions remain as to ordering of VLAN and
LLC.  Also, it's possible this same thing should be done as part of
ethernet.effective_ethertype.