sss_client: avoid leaking file descriptors
    
    If a pam or nss module is dlcolse()d and unloaded we were leaking
    the file descriptor used to communicate to sssd in the process.
    
    Make sure the fucntion used to close the socket file descriptor is
    called on dlclose()
    
    Silence autoconf 2.28 warnings (Patch by Jakub Hrozek)