fix logging flood created by setup_iface

The fix to setup iface causes a bit of a flood to the logging.  This
change uses dlog rather than flog to log output from setup_iface and
the loglevels have been changed from 3 to 4.
process.c: use dlog for RS on unknown interface

It's not unexpected for an unknown interface to get an RA.  This can
happen if the WAN gets RA's from the ISP.
add state_info.changed to avoid calling setup_iface too much

setup_iface will not be called over and over for interfaces that are not
ready with this change.  If an interface is marked as changes in netlink
or in set_interface_defaults, setup_iface will be called.