IFP: unify generated interfaces names
    
    Number of interfaces will grow. It is mandatory to unify names of
    generated structures and methods to simplify coding and debugging.
    
    The C name is created from D-Bus lowercased interface name using
    the following rewrite rules:
    org.freedesktop.sssd.infopipe -> iface_ifp
    . -> _
    
    Example:
    org.freedesktop.sssd.infopipe.Domains -> iface_ifp_domains
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>