seccomp: send process memory fd

seccomp: send process memory fd

There's an inherent race when reading a process's memory. The easiest way is to
have liblxc get an fd and check that the race was one, send it to the caller
(They are free to ignore it if they don't use recvmsg()).

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>