BSD: Fix router reachability tests

BSD: Fix router reachability tests

dhcpcd likes to indicate an on-link route as a NULL gateway.
However, we were testing this for an AF_LINK address to ensure
reachability. As such, it was possible to log a false positive.

Instead, record the existance of a valid AF_LINK address via
RTDF_GATELINK.

While here, ensure that we only treat RTM_DELETE as unreachable
and only other messages with RTDF_GATELINK as reachable, ignoring
other states so that a new incomplete address won't make us
unreachable if we're just discovering it.