PF_RING TODOs

[salcock]

The code for the PF_RING format still requires a bit of work to be ready for external use. Specifically:

• The zero-copy support is only partially written.
• The standard non-zero-copy support will need to be tested against a recent version of PF_RING -- I haven't run this code in 4 years so who knows what might have changed since then.
• The config_input functions will need to be updated to explicitly ignore all of the new options added in the past few years.
• Update docs to mention pfring: and describe how to use it (including installation of pfring libraries and drivers).
• Figure out how packaging is going to work -- is PF_RING support going to be in packaged libtrace? if so, we'll need to make sure that the users know to add the ntop repo to get access to the pfring packages.