Route validation

toggle RTF_UP depending on interface link state
on every link state change routing table needs to be traversed
expensive on large tables
easy to implement
make a linked list of routes per each interface 
additional pointers -> more memory needed
less expensive on large tables
my opinion is to use the first approach
rn_walktree() does most of the job already

needs lot of testing
some interfaces lie about their state
wireless cards show strange behaviour because of this