Patches to add flexibility to odhcpd's Route Info Option handling

Hi,

While working on a moderately complex IPv6 network setup, I ran into issues with odhcpd's handling of the RA Route Info Option. By default, odhcpd generates a Route Info Option in RAs based on the existence of unreachable routes on the router and this behavior cannot be changed. This pull request implements two new option which make it possible to configure odhcpd's behavior somewhat.

The option "ra_unreachable" makes it possible to disable the automatic Route Info Option. This is useful, for example, if the router contains multiple unreachable routes or if the route's prefix is different from what should be announced via RA.

The second option "ra_route" makes it possible to manually configure additional routes to be announced in /etc/config/dhcp. This is a per-interface option, different routes can be configured for different interfaces. It's a list option with the following value format:
  <IPv6/bits [low|medium|high]>
The preference field is optional (medium by default). Here's an example:
  list ra_route "fd00::/16 medium"

--Jan