Add sa6_checkzone_ifp() function.
It is similar to sa6_checkzone, but has additional argument - ifnet.
It is used for scope zone index initialization.
Use sa6_checkzone_ifp() function. Since we always know ifp, we can
use it to initialize sin6_scope_id if user didn't done this.
Check and initialize zone ids of src/dst addresses before calling
gif_set_tunnel. Also remove sa6_embedscope() calls.
Since we keep sin6_scope_id initialized, there is no need to recover it.
Remove in6_setscope() calls.
Remove in6_clearscope() calls and unused variable.
Replace unused argument off0 in tcp_signature_compute() and
tcp_signature_verify() functions to the pointer to struct in_conninfo.
It contains information about scope zone index of given tcp connection,
that is needed for SA allocation.