lxc-attach: elevate specific privileges

There are scenarios in which we want to execute process with specific
privileges elevated.

An example for this might be executing a process inside the container
securely, with capabilities dropped, but not in container's cgroup so
that we can have per process restrictions inside single container.

Similar to namespaces, privileges to be elevated can be OR'd:

    lxc-attach --elevated-privileges='CAP|CGROUP' ...

Backward compatibility with previous versions is retained. In case no
privileges are specified behaviour is the same as before: all of them
are elevated.