luci-base: Show multiple upstream interface

Several devices have multiple upstream interfaces, for example a fixed
and a mobile broadband connection. Currently, only one upstream
interface is shown per address family in Luci. So in my example, one of
the interfaces would not appear on the Status-page.

This PR introduces support for showing multiple upstream interfaces on
the Status-page. The code is not very complicated. get_status_by_route()
has been extended to return a list of all routes, and
get_wannet()/get_wan6net() now returns all upstream interfaces.

I could not find any other (active) users of these three functions than
calls triggered from the Status-page, so changing the default behavior
should be fine. get_wandev()/get_wan6dev() called get_status_by_route(),
but I could not find any place where those functions were called. I
removed the dev-functions instead of keeping the old
get_status_by_route().

On the status page, the wan/wan6-variables have been replaced with
arrays. When populating the html, we now iterate through these arrays
and create one element for each interface.

I have tested the code with different interface types, v4, v6, as well as
disconnecting and connecting interfaces. The status is updated and the
correct interfaces (or sometimes none at all) are shown.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>