RIPng: Modernize packet parsing style.

Lose the unused trace file props in struct rip6. Make rip6_entry_print()
void as its return value was discarded anyway; always fetch all fields of
RTE. In ripng_print() just tell rip6_entry_print() whether to print the
metric or not instead of fetching it; validate the version before
decoding the command, not after; consider an unknown command invalid; add
and use a standard invalid label. Lose all ND_TCHECK*() instances as they
are excess now. Update a test.