v5 of seccomp filter c/r patches

Hi all,

Here's v5 of the seccomp filter c/r set. The individual patch notes have
changes, but two highlights are:

* This series 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 the previous series and is a nicer API.

* I couldn't figure out a nice way to have SECCOMP_GET_FILTER_FD return the
  same struct file across calls, so we still need a kcmp command. I've narrowed
  the scope of the one being added to only compare seccomp fds.

Thoughts welcome,

Tycho