Add the ability to set `prefer_source' flag to an IPv6 address.
It affects the IPv6 source address selection algorithm (RFC 6724)
and allows override the last rule ("longest matching prefix") for
choosing among equivalent addresses. The address with `prefer_source'
will be preferred source address.

Obtained from:	Yandex LLC
MFC after:	1 month
Sponsored by:	Yandex LLC
Merge remote-tracking branch 'freebsd/master' into hardened/current/ptrace