MFC r271307:

MFC r271307:
  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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@272790 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f