lxc: Allow set_file_cap capability in container

This means you're able to set the bits for capabilities on files
inside the container. This is needed for e.g. many fedora packages
as they use finegrained capabilities rather than setuid binaries.

This is safe as we're not adding capabilities really, since the
container is already allowed to create setuid binaries. Setuid
binaries are strictly more powerful that any capabilities (as root implies
all capabilities).

This doesn't mean the container can *gain* capabilities that it
doesn't already have though. The actual set of caps are strictly
decreasing.