Add mkseccomp.pl, helper script to make seccomp profiles.

I was playing with the seccomp functionality of docker and it seems to work but the need to create the profiles using raw syscall numbers makes it more difficult than necessary. So I made myself a script which takes a list of syscalls and turns it into a seccomp profile. Here I'm submitting it for contrib, as I think others might find it useful.

Note the way this is done you could actually build a list of profiles for different applications, but I think that might be better done elsewhere. This is just the groundwork to get people started.