rtadvd: set route info lifetime to 0 during shutdown

[pouria]

Summary

RFC 4191 section 3.1: If the received route's lifetime
is zero, the route is removed from the Routing Table if present.

Therefore, tell hosts that those routes are not available anymore.
setting Router lifetime field to 0 is not enough since route info
directly consults routing table for nexthop and its not depends on
defrouter list.
Also, do not tell route information option is unknown.

Test Plan

configure a rtprefix option in rtadvd.conf,
then run:

rtadvd -sfDc /etc/rtadvd.conf -M vtnet0

followed by CTRL-C for graceful shutdown.
Since FreeBSD doesn't support RFC 4191 yet (I'm working on it)
verify functionality with tcpdump.

[pouria]

It also resolves one of the issues in PR263982.

[pouria]

fix missed skip.