slave: add host authorizations individually
    
    XAddHosts fails if any of the entries in the list fails.
    The gdm-initial-setup user isn't available on most systems,
    so XAddHosts fails on most systems.
    
    Upshot is, since commit 189cd818bf1592b5b8607793575bcf4e71ba8008
    most people have been relying on xauth cookies for X authorization.
    xauth cookies aren't nearly as robust across hostname changes, so
    that's not ideal.
    
    This commit calls XAddHost() 3 times instead of one XAddHosts
    call to work around the problem.  Ideally, we'd only add
    gdm-initial-setup when we need it instead of all the time.
    
    That will have to come later.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=686186
    (cherry picked from commit 2bd38e811f7f8194898ab4321eba36737729a493)