interfaces: immediately enable SLAAC during IPv6 initiation
interfaces: immediately enable SLAAC during IPv6 initiation

This improves PPPoE responsivenes for IPv6 acquire using
SLAAC on the "IPv4 interface".  Tests ok according to an
user and the impact on existing setups is low.

PR: https://forum.opnsense.org/index.php?topic=21506.0

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core