seccomp_ctx is already a void*, so don't use 'scmp_filter_ctx *'

Separately track the native arch from the arch a rule is aimed at.

Clearly ignore irrelevant architectures (i.e. arm rules on x86)

Don't try to load seccomp (and don't fail) if we are already
seccomp-confined.  Otherwise nested containers fail.

Make it clear that the extra seccomp ctx is only for compat calls
on 64-bit arch.  (This will be extended to arm64 when libseccomp
supports it).  Power may will complicate this (if ever it is supported)
and require a new rethink and rewrite.

NOTE - currently when starting a 32-bit container on 64-bit host,
rules pertaining to 32-bit syscalls (as opposed to once which have
the same syscall #) appear to be ignored.  I can reproduce that without
lxc, so either there is a bug in seccomp or a fundamental
misunderstanding in how I"m merging the contexts.

Rereading the seccomp_rule_add manpage suggests that keeping the seccond
seccomp context may not be necessary, but this is not something I care
to test right now.  If it's true, then the code could be simplified, and
it may solve my concerns about power.

With this patch I'm able to start nested containers (with seccomp
policies defined) including 32-bit and 32-bit-in-64-bit.

[ this patch does not yet add the default seccomp policy ]
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>