Tweak handling of HUP and new LLADDR. Just use expose and use sighup()

Tweak handling of HUP and new LLADDR. Just use expose and use sighup()
function. Don't exit dispatch() loop on SIGHUP, wait for privileged
child to execvp() dhclient. Eliminate struct imsg_hup since its
contents were not being used except in priv_cleanup(). And
'active_addr' works just as well there.

Cleaner and eliminates some extraneous log entries.