Add a chroot dir for the _rtadvd user.
Drop privs to the user _rtadvd after acquiring our socket.
When rc.d/rtadvd starts or reloads, the rtadvd config file is copied
into the chroot before starting or reloading rtadvd itself.
Create a symlink from /var/run/rtadvd.dump to the chroot
Inital idea from OpenBSD patch rtadvd.c r1.36
