Added PorpVariant and comments to explain how manual return works.

* Driven by trying to integrate ibGib with porpGib - specifically,
  integrating the more complex metaprogramming capabilities of ibPlan
  into the porpGib structure which only understands the "hardwired plan"
  that is GoToDestinationAndAutoReturn.
* Still need to implement handling the new porp variant in the node's
  send and receive procedures, as well as adjusting the continue method
  mechanism.

Signed-off-by: William Raiford <bill.mybiz@gmail.com>