If intra area prefixes move from one router to another router, cloning routes

If intra area prefixes move from one router to another router, cloning routes
may become gateway routes and contrary. The kernel does not allow to change the
flags RTF_GATEWAY / RTF_CLONING in RTM_CHANGE messages, but ospf6d tried this
anyway. The result is a broken route.
Instead of modifying such routes remove the old route and insert a new one.

Thanks to Raimund Specht for reporting the problem and testing the fix.

OK claudio@