[RFC] Introduce support for syscall filtering in containers

This PR introduces the ability to filter system calls on a per-container basis on Linux, using libseccomp to support multiple architectures.

This adds another layer of security between containers and the kernel. System calls which are unnecessary in a container or problematic from a security perspective can be restricted to prevent their use. Most of the truly problematic syscalls are already restricted by dropping capabilities; this adds an additional, finer-grained layer of protection.

There's a similar feature present in LXC already, with the significant difference that LXC uses a whitelist of system calls, whereas these patches use a blacklist. The blacklist approach ensures no difference in functionality to clients not explicitly aware of seccomp support (the restricted syscalls list in the container config is left empty, and the seccomp init function exits without taking action).

This PR adds a vendored library dependency (Go bindings for libseccomp) and a build dependency on libseccomp >= v2.1. The actual changes to libcontainer are fairly minimal, most of the delta is in the libseccomp bindings.

Presently missing: integration tests, documentation