Fix handling of scoped IPv6 addresses in IPSec code.

• in ipsec_encap() embed scope zone ids into link-local addresses in the new IPv6 header, this helps ip6_output() disambiguate the scope;
• teach key_ismyaddr6() use in6_localip(). in6_localip() is less strict than key_sockaddrcmp(). It doesn't compare all fileds of struct sockaddr_in6, but it is faster and it should be safe, because all SA's data was checked for correctness. Also, since IPv6 link-local addresses in the &V_in6_ifaddrhead are stored in kernel-internal form, we need to embed scope zone id from SA into the address before calling in6_localip.
• in ipsec_common_input() take scope zone id embedded in the address and use it to initialize sin6_scope_id, then use this sockaddr structure to lookup SA, because we keep addresses in the SADB without embedded scope zone id.

Differential Revision: https://reviews.freebsd.org/D2304
Reviewed by: gnn
Sponsored by: Yandex LLC