Scope related cleanup in rip6_output():

• remove sa6_embedscope() and in6_setscope() calls;
• rip6_output() calls in6_selectsrc, so it always has outgoing interface and we can use IPV6_USEROIF when calling ip6_output.
• add sa6_checkzone() to rip6_send. Application always must provide a proper scope zone id.