Restoring interface attributes

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

The thorpej-cfargs branch had a change to systematically remove
interface attributes from config attachments:

https://mail-index.netbsd.org/source-changes-hg/2021/03/23/msg273413.html

I don't understand what the purpose is.  It's like changing function
calls to not name the function that you're trying to call.  Now
there's no way to grep for, e.g., where wsmousedev children get
attached.  Why is this a good thing?

I would like to revert this change (adapted to the current API), and
go further in the opposite direction -- if a device has children at an
interface attribute, mandate the use of the interface attribute at the
call site, so that you can always find the places where children can
be attached at any given interface attribute in the source code.

Later, I'd like to make interface attributes have defined names (not
just strings that can have copypasta errors) and types so we have the
chance to detect mistakes at compile-time at the interface between
parent and child device code.