add RFC4191 support (#297)

add RFC4191 support (#297)

* add RFC4191 support

- handles route information options from RAs.
- refactor `sa_fromprefix()` to expose lower level functionality
- refactor `ipv6nd_rtprefix()` to be usable outside of `struct ra` context

* changes as requested by RM

- mostly minor/cosmetic changes
- functional change: "no longer a default router" warning moved to capture changes from routeinfo options

* simplify routeinfo_find/new