XFRM State Management API

current keys: (family, daddr, spi, proto)
not sufficient (especially for mobile ip)
userland daemon need to add/delete with specific source address
                struct xfrm_usersa_id {
                        xfrm_address_t  daddr;
                        __u32           spi;
                        __u16           family;
                        __u8            proto;
                        xfrm_address_t  saddr;     // NEW
                };
This is probably good for xfrm6_tunnel management, too.
We see "hashed" spi for xfrm6_tunnel.
Mobile IP is a kind of tunnel, anyway.