resolved: add an explicit way to configure whether a link is useful as default route

resolved: add an explicit way to configure whether a link is useful as default route

Previously, we'd use a link as "default" route depending on whether
there are route-only domains defined on it or not. (If there are, it
would not be used as default route, if there aren't it would.)

Let's make this explicit and add a link variable controlling this. The
variable is not changeable from the outside yet, but subsequent commits
are supposed to add that.

Note that making this configurable adds a certain amount of redundancy,
as there are now two ways to ensure a link does not receive "default"
lookup (i.e. DNS queries matching no configured route):

1. By ensuring that at least one other link configures a route on it
   (for example by add "." to its search list)

2. By setting this new boolean to false.

But this is exactly what is intended with this patch: that there is an
explicit way to configure on the link itself whether it receives
'default' traffic, rather than require this to be configured on other
links.

The variable added is a tri-state: if true, the link is suitable for
recieving "default" traffic. If false, the link is not suitable for it.
If unset (i.e. negative) the original logic of "has this route-only
routes" is used, to ensure compatibility with the status quo ante.