TODO

Here's my vague plan for further development:

1) Get rid of the "listening" arrays for TCP and UDP - they are a
waste of memory for something that's likely to be constant per sketch
and better expressed as code in program memory space.  Replace with a
function pointer.  We've a reasonable amount of stack space,
hopefully, and the compiler should be doing the tail-call
optimisation; when you register a new "listening" function, have it be
responsible for calling the previous one if the incoming connection /
packet isn't for us.

2) Simplify the UDP code.  The idea of a UDP "connection" may be left
over from someone who was used to thinking about sockets.  It's much
easier to pass incoming packets along a chain if listening functions
and see if anyone wants them.  Let any of these send UDP to wherever
they like too; no need for memory to store a "connection".

3) Separate DHCP and resolver out into separate modules - probably
worth keeping them in this library, but make it so that they have to
be invoked from the sketch.  Don't refer to them from NanodeUIP.cpp.

4) Sort out datatypes.  The Arduino Wiring environment defines some
perfectly good ones - byte, word, etc. - and we should be using these
instead of rolling our own.

5) Tidy up those awful comments!  Sure, the code to manual system is
nifty but we're not using it here!