Requesting comments on Multi-routing table usage

[Julian Elischer]

The current code in -current will add a new interface to all
FIBs.

So for example when you add a gre interface irt shows up everywhere.

This behaviour is probbaly correct for the base NICs on the system 
when you boot, but it is probably wrong in other cases.

For example, when mpd makes tunnels it probably
(but not always) wants to add that set of routes into one
FIB. Similarly for other apps that can create tunnels.

What is needed is a way to allow the caller to somehow
specify the behaviour wanted whenever new interfaces are added.

various things crossed my minds..
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Maybe real hardware shoudl go everywhere and virtual should go to
the FIB of the creator

Maybe P2P interfaces should not go everywhere.

Maybe a sysctl can be used to 'flip' teh mode from "everywhere"
to "specific fib" after boot has completed. (I have code for this but 
it's not the perfect solution).

Maybe ifconfig can set a new flag somewhere somehow.

Maybe a process can set a flag for itself saying what its mode is..
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The trouble is that there is not an "always correct" answer.
some people may want to see a tunnel turn up on all FIBs
and others may not.