PFPorts should rely on XML

[Spartan322]

Implementation PR: #130

PFPorts makes little sense in my eyes when we are using XML, I propose this format instead:

<PFPorts>
  <ssh /> <!-- this modifies the ssh default port -->
  <ftp Number="50" />
  <random_protocol Number="78">display name underscores are not spaces anymore</random_protocol>
  <!-- or -->
  <random_protocol Number="78" Display="display name underscores are not spaces anymore" />
  <!-- we could also do this even if its very verbose -->
  <random_protocol>
    <Number>78</Number>
    <Display>display name underscores are not spaces anymore</Display>
  </random_protocol>
  <ssh Remove="true" /> <!-- removes default port -->
</PFPorts>

While being a little more verbose, its guaranteed in the XML format, following the XML spec properly for definition behavior, and there is no special syntax to recall. If we really don't feel like introducing a breaking change and keep the old format, we can, it is slightly shorter in most cases, but for consistency I would say we should prefer this format.