net/{interfaces,netmon}: remove "interesting", EqualFiltered API

This removes a lot of API from net/interfaces (including all the
filter types, EqualFiltered, active Tailscale interface func, etc) and
moves the "major" change detection to net/netmon which knows more
about the world and the previous/new states.

Updates #9040

Change-Id: I7fe66a23039c6347ae5458745b709e7ebdcce245
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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