information schema parameter_default implementation

Here is an implementation of the
information_schema.parameters.parameter_default column.

I ended up writing a C function to decode the whole thing from the
system catalogs, because it was too complicated in SQL, so I abandoned
the approach discussed in [0].

[0]: http://archives.postgresql.org/message-id/1356092400.25658.6.camel@vanquo.pezone.net