v7 of seccomp filter c/r

Hi all,

Here's v7 of the seccomp filter c/r stuff. The biggest change is counting down
the seccomp tree instead of up for the index, although there are some other
small changes in the notes as well.

Note that this is now based on http://patchwork.ozlabs.org/patch/525492/ and
will need to be built with that patch applied. This gets rid of two incorrect
patches in a previous series and is a nicer API.

Thoughts welcome,

Tycho