Follow RFC 4443 p2.2 and always use own addresses for reflected ICMPv6
datagrams.

Previously destination address from original datagram was used. That
looked confusing, especially in the traceroute6 output.
Also honor IPSTEALTH kernel option and do TTL/HLIM decrementing only
when stealth mode is disabled.

Reported by: Marco van Tol <marco at tols org>
Reviewed by: melifaro
MFC after: 2 weeks
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D22631