This commit deals with different kernel and userspace layouts and nesting. Here
are three examples:
1. 64bit kernel and 64bit userspace running 32bit containers
2. 64bit kernel and 32bit userspace running 64bit containers
3. 64bit kernel and 64bit userspace running 32bit containers running 64bit containers
Two things to lookout for:
1. The compat arch that is detected might have already been present in the main
   context. So check that it actually hasn't been and only then add it.
2. The contexts don't need merging if the architectures are the same and also can't be.
With these changes I can run all crazy/weird combinations with proper seccomp
isolation.

Closes #654.

Link: https://bugs.chromium.org/p/chromium/issues/detail?id=832366Reported-by: Chirantan Ekbote <chirantan@chromium.org>
Reported-by: Sonny Rao <sonnyrao@chromium.org>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>