Start rtsold at boot if ipv6 autoconfig is requested.

This puts in a rtsold_enable="YES" line for every
interface, which is kind of hacky, but you aren't
supposed to enable ipv6 autoconfig on more than one
interface anyways, so we'll enforce that in the GUI.

Ticket: #3366

We'll have to deal with starting and stopping rtsold
on the fly as well.