SYSTEMD: Allow to use "/run" in ListenStream
    
    /var/run is only symbolic link to /run on some distributions
    and /run is mounted on tmpfs
    
    sh-4.4$ ls -l /var/run
    lrwxrwxrwx. 1 root root 6 Dec 12  2015 /var/run -> ../run
    
    Previously, we used $(localstatedir)/run in ListenStream
    which does not allow to use "/run" because we still need to
    store some files in under $(localstatedir) (/var).
    
    Autoconf 2.70 will add new configure time option --runstatedir
    for configuring runstatedir. ATM, we use just fallback implementation
    where $(runstatedir) is set to $(localstatedir)/run
    
    Merges: https://pagure.io/SSSD/sssd/pull-request/3691
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>