netlink: Indicate that ifinfomsg uses IFT_* types

[emaste]

Summary

On Linux struct ifinfomsg uses ARPHRD_* values, while we used IFT_*
types (which derive from SNMP ifType definitions in RFC 1573).
Update the header to reflect this.

PR:             272144

[pouria]

netlink specs clearly want to use arp types (ARPHRD_*). (See include/uapi/linux/rtnetlink.h of linux)
However, IMO, using arp types are not appropriate here, because we actually want to get the actual interface type ifp->if_type.
therefore our current implementation (IFT_*) is correct.

We also provide compatibility for linux.
For example we map our IFT_ETHER to ARPHDR_ETHER type in sys/compat/linux/linux_netlink.c:366.

This should be enough reasoning/evidence for PR272144.
Please add a note here to clarify that NOT using ARPHRD_* is intentional.
This should also close that PR too.

[pouria]

Don't forget to update the comment on the man page for rtnetlink(4).

[emaste]

Add note in manpage