Any code in mn is not usable by other Python code.

Hence, move this code into util, so other scripts can use it.

Now customConstructor, because it general to both links and nodes.

Thanks to James Zeng for pointing this out!

I think it's worth considering how we want to specify dpids for
switches. One way would be to have Mininet (optionally) pick them
automatically. Another way, which I have currently implemented, is
to intuit them from the name, for example s1 -> 1. The latter is
slightly inefficient, but is convenient because it ensures that
there is a logical mapping between switch names and dpids, which
is very helpful for debugging an OpenFlow system!

Probably we should just clarify that the easiest way to set a dpid
is to include it in the switch name, but you can also pass it in
as a custom parameter to the constructor.

Probably we should have a generic mechanism to specify OVS options.