Refactor the OpenFlow implementation.

Refactor the OpenFlow implementation.

This new implementation has an architecture that is much more suited to
eventually getting pushed upstream into the Linux kernel, because it does
not do any OpenFlow processing in the kernel.  Rather, we define a new
"datapath protocol" that secchan uses, via ioctl calls, to set up the
flow table in the kernel.

This implementation also should have much better performance with flows
that contain wildcards, since it uses a flow classifier that should be
much better than linear search in the cases that we suspect are important.

This release does contain some feature regressions; see the new file
MISSING at the root of the tree for more information.  We will be fixing
these regressions over the next weeks and months.

This has not been tested much.  It needs plenty of testing and QA before it
will be suitable for any kind of production environment.  The vswitchd
changes, in particular, have not been tested at all and thus vswitchd is
likely to be broken.