route: avoid parsing rt_msghdr2 fields as pid, seq, errno

[134130]

On Darwin, NET_RT_DUMP2 returns RTM_GET2 messages backed by rt_msghdr2. That header is not a superset of rt_msghdr: the offsets used by rt_msghdr for rtm_pid, rtm_seq, and rtm_errno are instead rtm_refcnt, rtm_parentflags, and rtm_use in rt_msghdr2.

The current classic parser unconditionally reads those offsets into RouteMessage.ID, RouteMessage.Seq, and RouteMessage.Err. Valid RIB entries with non-zero rtm_use are therefore reported as errors.

Represent pid, seq, and errno offsets in wireFormat and leave them unset for Darwin's rt_msghdr2 format. This fixes the incorrect Err value and avoids exposing rt_msghdr2 ref counts and parent flags through unrelated fields.

The inspected XNU route.h sources, from xnu-2422.1.72 through xnu-12377.101.15, use the same rt_msghdr2 layout and have no rtm_errno field. See bsd/net/route.h in apple-oss-distributions/xnu at those tags.

This change does not add API for rt_msghdr2-specific fields such as rtm_refcnt, rtm_parentflags, rtm_use, or rtm_inits. They need a separate API decision; preserving rtm_use via RouteMessage.Err would keep valid routes looking like failed operations.

Fixes golang/go#70593

Change-Id: I7011355e6b9643e7756557b34518b26c9f3e7749

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