net: provide list of available nets

[blueyed]

This makes it easier to loop over them and/or handle them separately.

[blueyed]

Should also be done for ./widgets/net_freebsd.lua then likely.

[McSinyx]

Documentation needs update. Although as a Python guy I think explicit is better than implicit, so maybe we should take the approach currently used in net_freebsd widget, i.e. match for '^' .. name .. ':' instead.

Edit: The explicit approach breaks backward compatibility however.