kern/60861: Make NPF routing syntax more robust and easily to work with old userland

["joe%ns1.eloisystems.com@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60861
>Category:       kern
>Synopsis:       Make NPF routing syntax more robust and easily to work with old userland
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Oct 08 07:50:00 +0000 2026
>Originator:     Emmanuel Nyarko
>Release:        NetBSD current
>Organization:
NetBSD
>Environment:
amd64 x86_64
>Description:

In our live NPF tests with routing on NetBSD current, we realized using old userland with updated kernel sends a missing nvlist parameter to the kernel. (because we changed the nvlist identifier from "route-interface" to "interface".
 only functional when users updated both userland and kernel. 

also, we can easily avoid passing "interface" parameter in configuration.
>How-To-Repeat:

run kernel with via and via6 and dup-to not present on top of userland without via and via6 parameters userland 
>Fix:

Yes