postgresql plugin: Added support for <Result> blocks.

postgresql plugin: Added support for <Result> blocks.

This is a replacement for the "Column" config option (which has been
deprecated). <Result> blocks allow for much more flexible handling of the
data. Besides being able to use data types with multiple data sources, the
plugin now supports to generate the type instance using information from the
database.

An example <Result> block might look like this (taken from the manpage):

  <Result>
    Type counter
    InstancePrefix "rt36_tickets"
    InstancesFrom "type"
    ValuesFrom "count"
  </Result>