netlink: specs: finish up operation enum-models

I had a (bright?) idea of introducing the concept of enum-models
to account for all the weird ways families enumerate their messages.
I've never finished it because generating C code for each of them
is pretty daunting. But for languages which can use ID values directly
the support is simple enough, so clean this up a bit.

"unified" model is what I recommend going forward.
"directional" model is what ethtool uses.
"notify-split" is used by the proposed DPLL code, but we can just
make them use "unified", it hasn't been merged :)

Signed-off-by: Jakub Kicinski <kuba@kernel.org>