FILES: Handle files provider sources
    
    Setup watches on passwd and group files provided with the files provider
    options passwd_files and group_files lists
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3402
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>