selinux: Only call semanage if the context actually changes
    
    https://fedorahosted.org/sssd/ticket/2624
    
    Add a function to query the libsemanage database for a user context and
    only update the database if the context differes from the one set on the
    server.
    
    Adds talloc dependency to libsss_semanage.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    (cherry picked from commit 1e0fa55fb377db788e065de917ba8e149eb56161)