Introduce new scope related functions.

• new macro to remove magic number - IPV6_ADDR_SCOPES_COUNT;
• sa6_checkzone() - this function checks sockaddr_in6 structure for correctness of sin6_scope_id. It also can fill correct value sometimes.
• in6_getscopezone() - this function returns scope zone id for specified interface and scope.
• in6_getlinkifnet() - this function returns struct ifnet for corresponding zone id of link-local scope.

Obtained from: Yandex LLC
Sponsored by: Yandex LLC