I have 3 different codes using the local_address6
value at a different place:

 - old "option-to" patch where if_register6() uses
  it in the if_register_socket() for global_v6_socket

 - new "parameter-identical" where send_packet6()
  uses it for the pktinfo->ipi6_addr

 - DHCPv4-over-IPv6 (rt28195b) branch where
  if_register_socket() binds to it

Only the last one provides a text for dhcpd.conf.5
which (incorrectly) explains the effect of the new
server option.

Have to think about what is needed (seems the
second option): implement and fix it...