backend: pass client Smack labels through internal API

To accomodate correct access checks for get_value priority logic, the
internal backend API needs access to the client Smack label.

For consistency, do the same thing for the set_value and unset_value
functions.

The access checks will be migrated in a future commit.
smack: move access checks to their proper locations

Now that the backend API has access to the client Smack labels, we can
do the Smack access checks here instead.