PostgreSQL type extensions for IP and MAC addresses.

I needed to record IP and MAC level ethernet addresses in a data
base, and I really didn't want to store them as plain strings, with
no enforced error checking, so I put together the accompanying code
as my first experiment with adding a data type to PostgreSQL.  I
then thought that this might be useful to others, both directly and
as a very simple example of how to do this sort of thing, so here
it is, in the hope that it will be useful.