MFC r238273 (by hrs):
  Remove "prefer_source" address selection option.  FreeBSD has had an
  implementation of RFC 3484 for this purpose for a long time and "prefer_source"
  was never implemented actually.  ND6_IFF_PREFER_SOURCE macro is left intact.

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.