add "fib reload" support to ospf6d/ospf6ctl, and trigger it

add "fib reload" support to ospf6d/ospf6ctl, and trigger it
automatically on a timeout after RTM_DESYNC (i.e. route socket
overflow). ported across from ospfd. fixes/ok claudio@

(it's much harder to overflow the routing socket buffer these
days since it was bumped to 2MB, but still happens occasionally
on a busy machine).